Using gsm codec with sip protocol

Hi all!
I can’t use gsm codec with sip protocol. I try’it with zoiper and x-lite, but no luck. I get in console this:

chan_sip.c:8508 process_sdp: No compatible codecs, not accepting this offer!

Softphones are configured only with gsm codec.
With iax protocol work well.
Any suggestion?

Please provide the relevant part of sip.conf and a trace of the SDP payloads on both sides of the SIP dialogue (asterisk sip set debug on output, although noisy, might help more).