Got it!
The problem had nothing to do with the pfSense box. Packets are whizzing around inside just fine.
With the LAN on one subnet (with SIP server) and the WLAN on another subnet, the problem is that the SIP registration ACK doesn't know how to get back to the WLAN wifi phone. A static route has to be added to the routing table on the SIP server box so that it knows the WLAN subnet can be found by going into the pfSense box through the LAN interface address.
When the LAN and WLAN were bridged, no additional SIP server route was needed.
It also helps to diagnose the problem with a Wireshark on the server box and not on the (LAN switch connected) browser box.
Thanks for your encouragement.
Bob G