Is it possible to change the trunk group credentials?, as there is no documentation about this in the wiki that I can find. Or in the web-interface settings.
It appears to me that I would be required to change the SIP trunk credentials of each trunk? Is that true?
What do you mean by no documentation? FreePBX doesn’t really have a concept of trunk groups. Outbound routes is where you select trunk order and perform LCR.
If you have a SIP peer then the credentials would be in the SIP trunk. Plenty of documentation between our site and Asterisk on how to configure SIP peers in Asterisk.
Perhaps if you told us more about your system (FreePBX, OS and Asterisk version and how it was installed) and exactly what you are trying to accomplish we could provide more specific recommendations.