Hey there,
I want to use a softphone on my android when I’m on the go.
The app I’m currently using is “Calls”, but this problem occurs with any SIP-Application.
Connections are established via an OpenVPN Tunnel to my pfSense appliance. The VPN-Subnet is configured as “local trusted” in FreePBX, all Ports to the PBX are allowed except 21,22,80,81&443 (TCP/UDP).
When the phone is connected to a WiFi-Network, everything works perfectly. Only when connected via LTE after exactly 32 seconds the connection drops. If read many posts about such behaviour but nothing helped yet. Do you have any advice for me on how to resolve this?
My experience with the number 32 is that a session in a router somewhere is timing out. Check the logs and see if there are messages about “failing to reply to our critical packet” are in there. That should help you get a handle on where the problem might be happening.
I’m currently suspecting my LTE-Provider to be the cause. They’re using NAT-Pools and I’m 99% sure they’re using SIP ALG. I’ve just texted them, we’ll see how it goes! 
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If your VPN is operating properly (SIP app is connecting via the VPN, freePBX sees the connection as coming from the address assigned by the pfSense server), that shouldn’t be possible. The data is encrypted and there should be no way the LTE provider could even tell it’s SIP.
Possibly, the SIP app is attempting NAT traversal and substituting a non-VPN IP address in the SIP or SDP. Looking at the traffic at the PBX end should tell you whether this is the case.
At the Asterisk command prompt, type
pjsip set logger on
or
sip set debug on
(if chan_sip extension)
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