diff --git a/openbsc/include/openbsc/gsm_data.h b/openbsc/include/openbsc/gsm_data.h
index bdcd0e0..b9cb48d 100644
--- a/openbsc/include/openbsc/gsm_data.h
+++ b/openbsc/include/openbsc/gsm_data.h
@@ -296,6 +296,10 @@
 
 	/* control interface */
 	struct ctrl_handle *ctrl;
+
+	/* Allow or disallow TCH/F on dynamic TCH/F_TCH/H_PDCH; OS#1778 */
+	bool dyn_ts_allow_tch_f;
+	/* TODO: vty for this; related: OS#1781 */
 };
 
 struct osmo_esme;
diff --git a/openbsc/src/libbsc/chan_alloc.c b/openbsc/src/libbsc/chan_alloc.c
index 1e5dd74..c3a7e0f 100644
--- a/openbsc/src/libbsc/chan_alloc.c
+++ b/openbsc/src/libbsc/chan_alloc.c
@@ -309,8 +309,9 @@
 			if (lchan)
 				type = GSM_LCHAN_TCH_F;
 		}
+
 		/* Try fully dynamic TCH/F_TCH/H_PDCH as TCH/F... */
-		if (!lchan) {
+		if (!lchan && bts->network->dyn_ts_allow_tch_f) {
 			lchan = _lc_dyn_find_bts(bts,
 						 GSM_PCHAN_TCH_F_TCH_H_PDCH,
 						 GSM_PCHAN_TCH_F);
diff --git a/openbsc/src/libbsc/net_init.c b/openbsc/src/libbsc/net_init.c
index ee0d445..6d03ee4 100644
--- a/openbsc/src/libbsc/net_init.c
+++ b/openbsc/src/libbsc/net_init.c
@@ -116,6 +116,8 @@
 	net->ext_max = GSM_MAX_EXTEN;
 	gsm_net_update_ctype(net);
 
+	net->dyn_ts_allow_tch_f = true;
+
 	return net;
 }
 
diff --git a/openbsc/src/osmo-nitb/bsc_hack.c b/openbsc/src/osmo-nitb/bsc_hack.c
index 0b360dc..e3e8927 100644
--- a/openbsc/src/osmo-nitb/bsc_hack.c
+++ b/openbsc/src/osmo-nitb/bsc_hack.c
@@ -299,6 +299,19 @@
 #endif
 	bsc_api_init(bsc_gsmnet, msc_bsc_api());
 
+	/*
+	 * For osmo-nitb, skip TCH/F for now, because otherwise dyn TS
+	 * always imply the possibility to have a mix of TCH/F and
+	 * TCH/H channels; if two phones request a TCH/F and a TCH/H,
+	 * respectively, they cannot call each other. If we deny TCH/F,
+	 * they will both fall back to TCH/H, and dynamic channels are
+	 * usable. See http://osmocom.org/issues/1778.
+	 *
+	 * A third-party MSC may well be able to handle a TCH/H TCH/F
+	 * mismatch.
+	 */
+	bsc_gsmnet->dyn_ts_allow_tch_f = false;
+
 	/* start control interface after reading config for
 	 * ctrl_vty_get_bind_addr() */
 	LOGP(DNM, LOGL_NOTICE, "CTRL at %s %d\n",