Asterisk has format and codec modules. If a format module exists, and both parties can handle the codec, the call will succeed but you may lose things like in-band tones and voice announcements. If the codec module also exists, Asterisk may be able to transcode.
I am not sure why Asterisk sent ringing before rejecting the codec. Maybe that version of Asterisk does know about GSM-EFR, but the phone doesn’t.
Hi,
I have logged from both ends of the system. The phone is a Nokia C2. I tried to interrogate its codec type via AT commands but it looks like this is not available. Wireshark logs show that the GSM phone will offer GSM-EFR. The X-Lite Softphone with GSM codec enabled is logged as GSM 06.10 (GSM-FR). The common denominator appears to be iLBC.
I get the feeling that to route a call, the signalling and voice channels are done separately. In that case, perhaps the SIP phone can ring but there is no voice available.
Cheers
Femto.
The call is being rejected, but only after the ringing.
Normally, for SIP calls, you don’t get to find the destination codec capabilities until it answers, athough you might expect it to reject the call immediately if it cannot send in one of the allowed codecs.
iLBC may be common, but iLBC is not being offered on the incoming call.
You really need the Asterisk debug and the SIP from both sides, to work out exactly what is going on codec wise, but my guess is that that version of Asterisk does recognize EFR, and the X-Lite is failing to reject the call at the earliest opportunity.
Note, there is the possibility of delayed offer in SIP. That is not happening inbound, and, unless it has changed, I don’t think that Asterisk uses it outbound, but it is possible to send INVITE without SDP and therefore codecs. If Asterisk did do late offer, Asterisk would not know there was a codec problem until the call was answered, and the X-Lite would not know until the call was fully up, and would have to immedaitely BYE it. I don’t think this is the case here.
Bingo! 
I now have a call from cell to SIP! At the moment it’s a bit of a workaround but it gives me a base to work from. So X-Lite has GSM codec selected (GSM-FR/GSM 06.10). The workaround was to restrict the BTS to only support (GSM-FR). The negotiation then settles at GSM-FR and the voice is established.
SIP to cell does not work yet … but that’s another story 
Cheers
Femto