This is new. IAX extension registers ok but calls bring this up. Has anyone else experienced this:
Sign up for a a beta copy of their H5 business editon. It’s actually pretty stable and you get 3 licenses to play around with. When your trial runs out you can download a new license automatically.
Check in your iax settings at freepbx which codecs are enabled. Change the sequence and have the same one in both your pbx and your softphone. Then try again and also post from your cli what asterisk pops up.
I had this same issue a couple of months ago. Changing the codecs under IAX settings didn’t fix it. I had to specify which codec I wanted in the extension settings for that Zoiper phone
disallow=all
allow=ulaw
For some reason, adding GSM in TRUNK settings for my outbound vendor IAX trunk did the trick.