diff --git a/include/osmocom/gsm/gsm0808_utils.h b/include/osmocom/gsm/gsm0808_utils.h
index 363fc39..34eae43 100644
--- a/include/osmocom/gsm/gsm0808_utils.h
+++ b/include/osmocom/gsm/gsm0808_utils.h
@@ -77,6 +77,7 @@
 int gsm0808_dec_cell_id_list(struct gsm0808_cell_id_list *cil,
 			     const uint8_t *elem, uint8_t len)
 			     OSMO_DEPRECATED("use gsm0808_dec_cell_id_list2 instead");
+int gsm0808_cell_id_list_add(struct gsm0808_cell_id_list2 *dst, const struct gsm0808_cell_id_list2 *src);
 int gsm0808_chan_type_to_speech_codec(uint8_t perm_spch);
 int gsm0808_speech_codec_from_chan_type(struct gsm0808_speech_codec *sc,
 					uint8_t perm_spch);
diff --git a/src/gsm/gsm0808_utils.c b/src/gsm/gsm0808_utils.c
index e4872b8..996456e 100644
--- a/src/gsm/gsm0808_utils.c
+++ b/src/gsm/gsm0808_utils.c
@@ -909,6 +909,85 @@
 	return (int)(elem - old_elem);
 }
 
+static bool same_cell_id_list_entries(const struct gsm0808_cell_id_list2 *a, int ai,
+				      const struct gsm0808_cell_id_list2 *b, int bi)
+{
+	struct gsm0808_cell_id_list2 tmp = {
+		.id_discr = a->id_discr,
+		.id_list_len = 1,
+	};
+	uint8_t buf_a[32 + sizeof(struct msgb)];
+	uint8_t buf_b[32 + sizeof(struct msgb)];
+	struct msgb *msg_a = (void*)buf_a;
+	struct msgb *msg_b = (void*)buf_b;
+
+	msg_a->data_len = 32;
+	msg_b->data_len = 32;
+	msgb_reset(msg_a);
+	msgb_reset(msg_b);
+
+	if (a->id_discr != b->id_discr)
+		return false;
+	if (ai >= a->id_list_len
+	    || bi >= b->id_list_len)
+		return false;
+
+	tmp.id_list[0] = a->id_list[ai];
+	gsm0808_enc_cell_id_list2(msg_a, &tmp);
+
+	tmp.id_list[0] = b->id_list[bi];
+	gsm0808_enc_cell_id_list2(msg_b, &tmp);
+
+	if (msg_a->len != msg_b->len)
+		return false;
+	if (memcmp(msg_a->data, msg_b->data, msg_a->len))
+		return false;
+
+	return true;
+}
+
+/*! Append entries from one Cell Identifier List to another.
+ * The cell identifier types must be identical between the two lists.
+ * \param dst[out]  Append entries to this list.
+ * \param src[in]   Append these entries to \a dst.
+ * \returns the nr of items added, or negative on error: -EINVAL if the id_discr mismatch
+ *   between the lists, -ENOSPC if the destination list does not have enough space. If an error is
+ *   returned, \a dst may have already been changed (particularly on -ENOSPC). Note that a return value
+ *   of zero may occur when the src->id_list_len is zero, or when all entries from \a src already exist
+ *   in \a dst, and does not indicate error per se. */
+int gsm0808_cell_id_list_add(struct gsm0808_cell_id_list2 *dst, const struct gsm0808_cell_id_list2 *src)
+{
+	int i, j;
+	int added = 0;
+
+	if (dst->id_list_len == 0
+	    && dst->id_discr != CELL_IDENT_BSS)
+		dst->id_discr = src->id_discr;
+	else if (dst->id_discr != src->id_discr)
+		return -EINVAL;
+
+	for (i = 0; i < src->id_list_len; i++) {
+		/* don't add duplicate entries */
+		bool skip = false;
+		for (j = 0; j < dst->id_list_len; j++) {
+			if (same_cell_id_list_entries(dst, j, src, i)) {
+				skip = true;
+				break;
+			}
+		}
+		if (skip)
+			continue;
+
+		if (dst->id_list_len >= ARRAY_SIZE(dst->id_list))
+			return -ENOSPC;
+
+		dst->id_list[dst->id_list_len++] = src->id_list[i];
+		added ++;
+	}
+
+	return added;
+}
+
 /*! Convert the representation of the permitted speech codec identifier
  *  that is used in struct gsm0808_channel_type to the speech codec
  *  representation we use in struct gsm0808_speech_codec.
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index a6ea47d..d99121e 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -176,6 +176,7 @@
 gsm0808_enc_cell_id_list2;
 gsm0808_dec_cell_id_list;
 gsm0808_dec_cell_id_list2;
+gsm0808_cell_id_list_add;
 gsm0808_chan_type_to_speech_codec;
 gsm0808_speech_codec_from_chan_type;
 gsm0808_speech_codec_type_names;
diff --git a/tests/gsm0808/gsm0808_test.c b/tests/gsm0808/gsm0808_test.c
index 9730be2..f467099 100644
--- a/tests/gsm0808/gsm0808_test.c
+++ b/tests/gsm0808/gsm0808_test.c
@@ -28,6 +28,7 @@
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>
+#include <errno.h>
 
 #define VERIFY(msg, data, len) 						\
 	if (msgb_l3len(msg) != len) {					\
@@ -1063,6 +1064,198 @@
 	msgb_free(msg);
 }
 
+static void print_cil(const struct gsm0808_cell_id_list2 *cil)
+{
+	unsigned int i;
+	if (!cil) {
+		printf("     cell_id_list == NULL\n");
+		return;
+	}
+	switch (cil->id_discr) {
+	case CELL_IDENT_WHOLE_GLOBAL:
+		printf("     cell_id_list cgi[%u] = {\n", cil->id_list_len);
+		for (i = 0; i < cil->id_list_len; i++)
+			printf("       %2d: %s\n", i, osmo_cgi_name(&cil->id_list[i].global));
+		printf("     }\n");
+		break;
+	case CELL_IDENT_LAC:
+		printf("     cell_id_list lac[%u] = {\n", cil->id_list_len);
+		for (i = 0; i < cil->id_list_len; i++)
+			printf("      %2d: %u\n", i, cil->id_list[i].lac);
+		printf("     }\n");
+		break;
+	case CELL_IDENT_BSS:
+		printf("     cell_id_list bss[%u]\n", cil->id_list_len);
+		break;
+	case CELL_IDENT_NO_CELL:
+		printf("     cell_id_list no_cell[%u]\n", cil->id_list_len);
+		break;
+	default:
+		printf("     Unimplemented id_disc\n");
+	}
+}
+
+void test_cell_id_list_add() {
+	const struct gsm0808_cell_id_list2 cgi1 = {
+		.id_discr = CELL_IDENT_WHOLE_GLOBAL,
+		.id_list_len = 1,
+		.id_list = {
+			{
+				.global = {
+					.lai = {
+						.plmn = { .mcc = 1, .mnc = 2, .mnc_3_digits = false },
+						.lac = 3,
+					},
+					.cell_identity = 4,
+				}
+			},
+		},
+	};
+
+	const struct gsm0808_cell_id_list2 cgi2 = {
+		.id_discr = CELL_IDENT_WHOLE_GLOBAL,
+		.id_list_len = 2,
+		.id_list = {
+			{
+				.global = {
+					.lai = {
+						.plmn = { .mcc = 1, .mnc = 2, .mnc_3_digits = true },
+						.lac = 3,
+					},
+					.cell_identity = 4,
+				}
+			},
+			{
+				.global = {
+					.lai = {
+						.plmn = { .mcc = 5, .mnc = 6, .mnc_3_digits = true },
+						.lac = 7,
+					},
+					.cell_identity = 8,
+				}
+			},
+		},
+	};
+
+	const struct gsm0808_cell_id_list2 cgi2a = {
+		.id_discr = CELL_IDENT_WHOLE_GLOBAL,
+		.id_list_len = 2,
+		.id_list = {
+			{
+				.global = cgi2.id_list[0].global
+			},
+			{
+				.global = {
+					.lai = {
+						.plmn = { .mcc = 9, .mnc = 10, .mnc_3_digits = true },
+						.lac = 11,
+					},
+					.cell_identity = 12,
+				}
+			},
+		},
+	};
+
+	const struct gsm0808_cell_id_list2 cgi3 = {
+		.id_discr = CELL_IDENT_WHOLE_GLOBAL,
+		.id_list_len = 2,
+		.id_list = {
+			{
+				.global = {
+					.lai = {
+						.plmn = { .mcc = 13, .mnc = 14, .mnc_3_digits = true },
+						.lac = 15,
+					},
+					.cell_identity = 16,
+				}
+			},
+			{
+				.global = {
+					.lai = {
+						.plmn = { .mcc = 16, .mnc = 17, .mnc_3_digits = true },
+						.lac = 18,
+					},
+					.cell_identity = 19,
+				}
+			},
+		},
+	};
+
+
+	const struct gsm0808_cell_id_list2 lac1 = {
+		.id_discr = CELL_IDENT_LAC,
+		.id_list_len = 1,
+		.id_list = {
+			{
+				.lac = 123
+			},
+		},
+	};
+
+	const struct gsm0808_cell_id_list2 lac2 = {
+		.id_discr = CELL_IDENT_LAC,
+		.id_list_len = 2,
+		.id_list = {
+			{
+				.lac = 456
+			},
+			{
+				.lac = 789
+			},
+		},
+	};
+
+	struct gsm0808_cell_id_list2 cil = {};
+
+	printf("------- %s\n", __func__);
+
+	print_cil(&cil);
+
+#define ADD_QUIET(other_cil, expect_rc) do { \
+		int rc = gsm0808_cell_id_list_add(&cil, &other_cil); \
+		printf("\ngsm0808_cell_id_list_add(&cil, &" #other_cil ") --> rc = %d\n", rc); \
+		OSMO_ASSERT(rc == expect_rc); \
+	} while(0)
+
+#define ADD(other_cil, expect_rc) ADD_QUIET(other_cil, expect_rc); print_cil(&cil)
+
+	ADD(lac1, 1);
+	ADD(lac1, 0);
+	ADD(lac2, 2);
+	ADD(lac2, 0);
+	ADD(cil, 0);
+	ADD(cgi1, -EINVAL);
+
+	printf("\ncan't add to BSS list\n");
+	cil.id_list_len = 0;
+	cil.id_discr = CELL_IDENT_BSS;
+	print_cil(&cil);
+	ADD(lac1, -EINVAL);
+
+	printf("\nother types (including NO_CELL) take on new type iff empty\n");
+	cil.id_list_len = 0;
+	cil.id_discr = CELL_IDENT_NO_CELL;
+	print_cil(&cil);
+	ADD(cgi1, 1);
+	ADD(cgi1, 0);
+	ADD(cgi2, 2);
+	ADD(cgi2, 0);
+
+	cil.id_list_len = GSM0808_CELL_ID_LIST2_MAXLEN - 1;
+	printf("\ncil.id_list_len = %u", cil.id_list_len);
+	ADD_QUIET(cgi2a, 1);
+	printf("cil.id_list_len = %u\n", cil.id_list_len);
+
+	cil.id_list_len = GSM0808_CELL_ID_LIST2_MAXLEN - 1;
+	printf("\ncil.id_list_len = %u", cil.id_list_len);
+	ADD_QUIET(cgi3, -ENOSPC);
+	printf("cil.id_list_len = %u", cil.id_list_len);
+	ADD_QUIET(cgi2a, -ENOSPC);
+	printf("cil.id_list_len = %u\n", cil.id_list_len);
+
+	printf("------- %s done\n", __func__);
+}
+
 int main(int argc, char **argv)
 {
 	printf("Testing generation of GSM0808 messages\n");
@@ -1103,6 +1296,8 @@
 	test_gsm0808_enc_dec_cell_id_list_multi_lac_and_ci();
 	test_gsm0808_enc_dec_cell_id_list_multi_global();
 
+	test_cell_id_list_add();
+
 	printf("Done\n");
 	return EXIT_SUCCESS;
 }
diff --git a/tests/gsm0808/gsm0808_test.ok b/tests/gsm0808/gsm0808_test.ok
index e101d65..a049dec 100644
--- a/tests/gsm0808/gsm0808_test.ok
+++ b/tests/gsm0808/gsm0808_test.ok
@@ -19,4 +19,89 @@
 Testing creating Paging Request
 Testing creating DTAP
 Testing prepend DTAP
+------- test_cell_id_list_add
+     cell_id_list cgi[0] = {
+     }
+
+gsm0808_cell_id_list_add(&cil, &lac1) --> rc = 1
+     cell_id_list lac[1] = {
+       0: 123
+     }
+
+gsm0808_cell_id_list_add(&cil, &lac1) --> rc = 0
+     cell_id_list lac[1] = {
+       0: 123
+     }
+
+gsm0808_cell_id_list_add(&cil, &lac2) --> rc = 2
+     cell_id_list lac[3] = {
+       0: 123
+       1: 456
+       2: 789
+     }
+
+gsm0808_cell_id_list_add(&cil, &lac2) --> rc = 0
+     cell_id_list lac[3] = {
+       0: 123
+       1: 456
+       2: 789
+     }
+
+gsm0808_cell_id_list_add(&cil, &cil) --> rc = 0
+     cell_id_list lac[3] = {
+       0: 123
+       1: 456
+       2: 789
+     }
+
+gsm0808_cell_id_list_add(&cil, &cgi1) --> rc = -22
+     cell_id_list lac[3] = {
+       0: 123
+       1: 456
+       2: 789
+     }
+
+can't add to BSS list
+     cell_id_list bss[0]
+
+gsm0808_cell_id_list_add(&cil, &lac1) --> rc = -22
+     cell_id_list bss[0]
+
+other types (including NO_CELL) take on new type iff empty
+     cell_id_list no_cell[0]
+
+gsm0808_cell_id_list_add(&cil, &cgi1) --> rc = 1
+     cell_id_list cgi[1] = {
+        0: 001-02-3-4
+     }
+
+gsm0808_cell_id_list_add(&cil, &cgi1) --> rc = 0
+     cell_id_list cgi[1] = {
+        0: 001-02-3-4
+     }
+
+gsm0808_cell_id_list_add(&cil, &cgi2) --> rc = 2
+     cell_id_list cgi[3] = {
+        0: 001-02-3-4
+        1: 001-002-3-4
+        2: 005-006-7-8
+     }
+
+gsm0808_cell_id_list_add(&cil, &cgi2) --> rc = 0
+     cell_id_list cgi[3] = {
+        0: 001-02-3-4
+        1: 001-002-3-4
+        2: 005-006-7-8
+     }
+
+cil.id_list_len = 126
+gsm0808_cell_id_list_add(&cil, &cgi2a) --> rc = 1
+cil.id_list_len = 127
+
+cil.id_list_len = 126
+gsm0808_cell_id_list_add(&cil, &cgi3) --> rc = -28
+cil.id_list_len = 127
+gsm0808_cell_id_list_add(&cil, &cgi2a) --> rc = -28
+cil.id_list_len = 127
+------- test_cell_id_list_add done
 Done