I configured a soft phone to use gsm, and I specified gsm as one of the codecs supported by SIP phones. But when I try to make a call from a channel using slin (aka L16), asterisk writes warnings that it cannot convert from sln to gsm; the verbatim messages are:
Unable to find a codec translation path from (slin) to (gsm)
Asked to transmit frame type gsm, while native formats is (slin) read/write = slin/slin
Codec mismatch on channel <foo>;1 setting write format to gsm from slin native formats (slin)
FWIW, there is no problem when I configure the softphone to use PCMU rather than GSM.
The menuselect configuration tool shows that I have codec_gsm is disabled because it depends on gsm(E), which I have not been able to figure out how to create.
If there is existing doc or a forum entry that describes how to do this, please point me to it.