This is a known limitation of chan_sip trunks if you have multiple registered trunks to the same host. When the provider sends a call (a SIP INVITE) to your PBX, Asterisk looks at the source IP of the INVITE and matches it against the IPs of the registered configured trunks. If there are multiple matching IPs, it will just take the first one it sees.
There is a parameter in pjsip (the name of which I have completely forgotten) which addresses this issue, but IIRC it is not supported yet in FreePBX. Since I can’t recall the parameter, I can’t search for an open ticket, but I am pretty sure there is a feature request on this in our issue tracker.
edit - Remembered, it’s the pjsip line parameter, and its available now in ver. 14 only.
https://issues.freepbx.org/browse/FREEPBX-14812