Hi Ryan,
Where are your trunks !?
if your provider can just send calls to your IP address then just do IP based authentication in Kamailio and once provider is authenticated relay the call to the Internal PBX.
1 - allow IP AUTHENTICATION by adding line
#define WITH_IPAUTH
after the line saying "#define WITH_AUTH"
2 - Put the IP address plus port of the provider in "permission" database table and restart Kamailio (for first time only) for next time you make changes in that table execute this command
Linux:~#kamctl address reload
3 - Now everytime your provider sends a call it will be accepted BUT the call still needs to be routed to the internal PBX.
4 - since WITH_ASTERISK is defined on top as well so Kamailio will check the IP address of your internal PBX from this:
asterisk.bindip = "192.168.178.25" desc "Asterisk IP Address"
asterisk.bindport = "5080" desc "Asterisk Port"
If you want to have a different criteria to route call to internal PBX like Load-Balancing or decide based on DID the calls goes to a specific server, or based on accound it routes to a specific PBX then thats your logic and should be handled inside the route[TOASTERISK] - similarly route[FROMASTERISK] needs changes to allow calls coming back from Internal PBXs.
I hope it just gives you some idea of what to do next.
Regards,
Sammy