Obscure VOIP bug on Verizon/Motorola phones.

For the past week, I was trying to figure out why the ZOIPER VOIP application on android didn’t work
on WIFI to my VOIP PBX but worked when it was on the 4G Data network.
My own phone LG S3 worked fine with the same kind of SIP extension.
I made sure STUN worked (used for going through firewalls), and it worked fine with my device, just not his.
I thought it might be the Wireless hardware, the router, even the Xfinity cable internet service.
I had the device even tested on other wireless networks with other providers, still nothing.
After this, I had a laptop loaded up with ZOIPER and a new VOIP/SIP extension and it worked.
Then we did a google search on something like “verizon moto does not work with zoiper”
and turned up an obscure article on this…
Cannot connect to SIP Servers when on WiFi. When on LTE, there are no issues.
It turns out, one of the premium “features” (translation: BUG) in some of the newer Verizon
smartphones like Pixel, allows the phones to make a phone call through WIFI if you don’t have 4G/Cell service
available. This “feature” blocks 5060 (SIP) port from making a connection, needed for standard
SIP VOIP phones. This “feature” has been known about since November 2016 and yet, there is
no addressing of this issue by Verizon or Motorola.
You have to change the port SIP runs on to something else to make it work.
Luckily, ZOIPER supports IAX2 (port 4569), once we created an extension, it works.
It doesn’t even need STUN to traverse a NATTED firewall. It just works.