Allow specific SIP connection FreePBX

Hey I am using FreePBX with firmware: 5.211.65-14 and service pack 1.0.0.0

On this asterisk server I have everything up and running, but inbound phone calls might be rejected:

WARNING[56522][C-00000ba4]: Ext. s:6 @ from-sip-external: "Rejecting unknown SIP connection from xxx.xx.x.xx

I talked to my service provider and they said this happens because they use a sip proxy, so my trunk may have connected to server A, but an incomming phone call might be offered by server B.
In this case it is rejected.

Now I am trying to find a solution for this problem, but could not find one so far.
I tried setting the 'insecure' to very.
I tried adding the ip's of both servers in the whitelist of the firewall.
I added the ip's of both servers in the peer details using 'permit='

Do you know how to allow both ip's while connected to just one of those?

Thanks in advance.