I'm not sure that this is a pfSense problem, but probably more of a general VPN VoLTE problem. I have my VPN configured to not route all traffic through the VPN, it only sends the traffic on my work network through the VPN. Public internet still goes out on the regular public gateway.
I've got AT&T and called them today to try to troubleshoot the issue. The lady was surprising knowledgeable and actually reached out to their Tier 3 when she was on the phone with me. He did a call trace and said it looked like a device issue not a network issue so they referred me to Samsung. They offered to conference the call with them, but I had to jump on a conference call.
If you have any success or further input on what you found out about this I'd love to hear it.
My guess is that the call signaling still goes out over the regular voice network and they just reroute the audio payload over the data network. My phone will initiate the call, but it usually drops before the other side ever even rings; at most I get 1 unanswerable ring.