We can wrestle that bear later. My suspicion is that your config would be a lot easier with PJ-SIP.
In your current config, instead of “type=peer” use these settings:
This sets up your inbound to use the settings in your chan-sip setup. The x.x.x.x is the IP address of the server. If there’s only one, you only need one trunk, but you need one for each IP address of the ITSP phone server. If they have two, you need two trunks. If they have 50, you need 50 trunks (something that PJ-SIP simplifies).