I have Kamailio on a VPS using sip, and I also use another sip provider: didlogic.net
T-Mobile is blocking didlogic.net according to my tests. (I plan to call them and raise hell)
My VPS sip server is not blocked by T-Mobile.
I turn on OpenVPN on my Android, and didlogic.net is still blocked. With out spending a bunch of time debugging and tracing I assume Zoiper is not using the OpenVPN connection? All my other stuff stuff (browser/ apps) runs though the vpn. If I turn off OpenVPN and use wifi, or att sim card, I can connect to didlogic.net.
Also: I can see in my Kamailio logs that I am getting logs of phone calls made that are not sip calls, but cell calls. This should not happen. Your app is basically telling the sip provider a detail of all calls incoming and outgoing regardless if it is sip or not. I have noticed this for a long time.