I installed Zoiper on my Note 8 and Tablet. When connected to my wifi Zoiper has no problem navigating the IVR on my Freepbx system. If I connect via VPN from outside my network neither device manages to pass DTMF tones. I can call into the voicemail and make/receive calls locally or remotely.only DTMF do not work over VPN.
There aren't any weird firewall rules setup on my firewall for the VPN. I'm using a Ubiquiti USG Pro 4.
MizuDroid works fine over the same VPN and devices passing DTMF. The bluetooth doesn't work on it so I tried Zoiper. Zoiper bluetooth works flawlessly. Now if I can get DTMF to work.