Internet service and VoIP

We've had VoIP phones running on a few T-Mobile Home Internet devices for over a year (we do not use the internal WiFi on the gateway - we use independent mesh routers, which shouldn't matter).  Within the last month, our VoIP phones stopped working.  After HOURS of trying to figure out why (and a long call with T-Mobile customer care that had no idea why VoIP wouldn't work anymore), I discovered that our TMHI gateway had spontaneously started blocking Port 5060, which is most commonly used for SIP (VoIP) traffic.  Changing our VoIP port (to 5062 in our case) solved the issue immediately.  The available alternative ports you can use for VoIP differ, depending on your VoIP carrier.

Two days ago, our second TMHI gateway started blocking Port 5060.  So, I had to switch the port on those phones as well.  It appears T-Mobile pushed an update to at least some of their gateways (ours was an Arcadyan) that caused this inexplicable issue.  I HAVE NO IDEA WHY, and apparently neither does customer service.  I spent hours and hours trying to fix it and it was pretty frustrating.

Now my worry is, if they are going to start randomly blocking ports used by SIP, will they eventually block the new ports we are using?  We depend on our VoIP phones for our business.  If they keep doing this, we are going to have to dump T-Mobile.  They need to fix it.  If they are going to market themselves as a legitimate internet service provider, they need to act like one and stop doing things like this.