Specific Audio Format - How to set?


I´m doing some tests with zaptel, SIP, IAX and my own channell. I want to know if I can tell to asterisk to use a specific audio format.

As far I could understand, wave linear format is always used if it´s available. Testing with a softphone, I need to uncheck all formats and keep only GSM format checked to Asterisk use it.

My card/channell has native support to wave slinear, ulaw, alaw and gsm but if I pass all these formats in my initialization and make a call to, e.g., digium card, asterisk will use SLINEAR but I want to “force” it to use GSM. I only achieve this if I tell to asterisk that my card supports only gsm…

Any help will be appreciated.