PfSense VoIP issue

I have the weirdest problem I ever faced in pfSense, and after some full days of debugging I think I need some help. I have a remote SIP/PBX service/account and a local VoiP client (wifi phone, Grandstream WP810). The phone is connected to interface MGMT with no blocking firewall rules. When I call from outside to the phone, it rings, but when I click answer on the phone it continues to ring.

Both the phone and SIP account are definitely working since I tested it directly behind a FritzBox router (and set the phone as an exposed host in the GUI, which means fully open to the internet). Than the connection/call just works. So the problem seems pfSense firewall related.

I already tried all the proposed settings on the internet/docs and fora, like port 5060 outbound NAT, conservative firewall etc etc.

Even stranger is that when I use a Windows SIP client (e.g. microSIP) instead of the phone, it works behind pfSense!

This is the difference in flows (in Wireshark):

Left the phone behind pfSense, right the Windows microSIP client behind pfSense.

Anyone a clue? All help appreciated!