FreePBX 408 timeout error with VPN

I configured my phone to work with my home PPTP VPN and it’s working fine. Then, I installed community Zoiper app on the same mobile and configured the settings and Zoiper registered with my home FreePBX. I can now call my home IP extension while outside and on the road.

But suddenly, I’m not able to register my Zoiper extension and I’m receiving 408 error. Nothing had been changed with my FreePBX settings but I’m getting 408’s.

I can still register when I’m connected on WiFi at home, even with VPN activated.

What could be the problem?

Is it possible for a provider to block SIP ports even when on VPN connection?