Collect network evidence when phones lose service

When phones fail after a network change, compare one affected device with one working device on the same intended service. Record the time of each observation and the exact switch or access point used.

Check the first differences

Compare IP address, subnet mask, default gateway, VLAN, DHCP server and option values. Then test DNS resolution of the call-server name from the voice network. A correct address with a wrong gateway or stale DNS answer can stop registration.

Follow the packets

If basic reachability works, inspect the registration attempt and response at the phone or network boundary. For connected calls with missing audio, check the media path separately. Preserve the original lease, switch and packet records before rebooting equipment.

Escalate with the observed differences and a limited capture. Avoid sharing complete configurations that contain credentials.

The voice networking cheat sheet explains the fields being compared.