GSM codec trunk config?

Hi, I’m using the FreePBX distro with who supports GSM. I’m using gsm as a codec for the low bandwidth benefits and to avoid paying for g.729. I have allow=gsm in my trunk and in Asterisk SIP settings I have gsm enabled and dragged to the top. The trunk seems to be connecting, but when I dial out on my Yealink phone I get a “All Circuits are Busy” error. The phones don’t have GSM support, but I imagine Asterisk should be transcoding ulaw into GSM for them.

If GSM is touchy or unsupported, I’m open to g.729 but would like to get a free/test license first to see if it really brings in any quality difference compared to GSM. So far, I haven’t been able to find this.

edit: Error in the logs: NoOp(“SIP/5840-00000022”, “TRUNK Dial failed due to CHANUNAVAIL HANGUPCAUSE: 58 - failing through to other trunks”) in new stack