RTP Is Not SIP, and That Difference Explains a Lot of Broken Calls
Signaling and media usually take different paths, use different ports and fail for different reasons, so a working SIP registration says very little about RTP health.
UDP
UDP Was a Deliberate Starting Point for the PBX MVP
Transport choice should match the test objective before adding more complexity.
DHCP Is More Than Automatic IP
DHCP is easier to troubleshoot when treated as a timed client-server exchange that delivers an address plus the parameters a host needs to participate in the network.