diff --git a/src/mtp2_iac.erl b/src/mtp2_iac.erl
index d23aaf0..1b756b7 100644
--- a/src/mtp2_iac.erl
+++ b/src/mtp2_iac.erl
@@ -146,6 +146,10 @@
 not_aligned(emergency, LoopDat) ->
 	% mark emergency
 	{next_state, not_aligned, LoopDat#iac_state{emergency=1}};
+not_aligned(si_os, LoopDat) ->
+	% ignore SIOS in this state, as some implementations (notably
+	% yate) seem to send it in violation of the spec
+	{next_state, not_aligned, LoopDat};
 not_aligned({timer_expired, t2}, LoopDat) ->
 	% send 'alignment not possible' to LSC
 	send_to_lsc(alignment_not_possible, LoopDat),