I use thinktel for a voip provider. For host / IP if I enter edm.trk.tprm.ca (which resolves to 18.104.22.168) then when I place a call my INVITE packet gets sent to 22.214.171.124 as expected. But Thinktel wants to tighten their security and since the host name is inserted into the INVITE request line, this causes problems for thinktel. They want me to change my sip server setting to the IP instead of hostname. When I use 126.96.36.199, 3cx will then use 188.8.131.52 to place outbound calls. This .142 number is an internal IP for thinktel. The only reference I can find to .142 is in the SIP record-route header. I can't figure out why 3cx is choosing to use that instead of .52.