Iax2 codec behavior

My pbx has two Iax2 trunks

pbx A ---- GSMcodec ----- pbx B
pbx A ---- ALAWcodec ---- pbx C

When pbx C calls pbx A extensions it gets “all circuit busy” message, Iax debug says : Unable to negotiate codec.

I got it working by:
1 disable trunk between pbx A and pbx B or
2 use the same codec on both trunks

Is this an expected behavior ?
Should multiple Iax2 trunks on same pbx use the same codec ?

Whilst in theory using 2 codecs could work, although I’ve not tested myself. It would be better to run as much as possible on the same codec to avoid transcoding which, unless you have a transcoding card, will put pressure on the PBX for no reason. So all the phones, IAX trunks and external trunks just use a single codec.

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