Every time I transfer a call, I hear pbx-transfer.gsm. Is there any way that I can change this message to play something else? Thanks
Yes replace it with an alternative recording. Asterisk can be used to make the recording.
Note that it plays pbx-transfer.X, not specifically pbx-transfer.gsm. Asterisk has CLI commands to convert between formats, but the preceding fact means that if you in a Microsoft house, you can record a file in .wav format, as long as you make mono, 16bit, 8kHz. You must use lower case wav for Asterisk.
Asterisk recognizes .WAV and .wav49 as meaning Windows .wav files recorded in GSM format at 8kHz. I believe sound recorder supports this. These differ from .gsm only in that they have some extra meta-data.
Asterisk will choose the format that is cheapest to convert to the codec being used by the phone. Default installations of Asterisk use gsm to keep the sound file sizes down, not because it is the best format for anyone not using GSM as the phone codec.
Do you know what config file I change this in? Or I would even overwrite the file if I could find the location.
You have to overwrite the file (or delete it and add one in an alternative format).
Use the locate command to find it, or use find, if locate has not be fully installed.