Multi-Carrier on an iPhone 12 Max Pro

by Colin Berkshire

Imagine a cellular plan where your phone roamed to the best, closest cellular tower, without regard to what carrier owned it.

Well, I have achieved that.

My phone naturally is on the T-Mobile network. (I recently switched from Verizon because of increasingly crappy coverage.)

But when I am in a bad spot on the T-Mobile network my phone smoothly moves over to Verizon.

Plus I have all of these features:

  • WiFi calling when on WiFi.
  • Roam across T-Mobile or Verizon, transparently.
  • Personal phone number and office extension on my phone, concurrently. (And, any number of lines, actually.)
  • Almost free worldwide calling. Free calling back to the US.

Here’s how I did it:

  1. You need a Dual-SIM phone, such as iPhone 12.
  2. Subscribe to data service on T-Mobile.
  3. Subscribe to data service on Verizon.
  4. With iPhone 12, open carrier will be a physical SIM and one will be an eSIM (Dual concurrent SIM)
  5. Subscribe to a VOIP service, like your office phone or
  6. Use a SIP client on your smartphone, such as Groundwire.

Incoming calls go to your VOIP carrier (such as and then route via the internet to your phone’s SIP client. If on WiFi the routing will be via WiFi. Otherwise, the call will route over one or the other cellular company. Your iPhone can choose which one to use as the primary carrier and can switch to the alternate carrier as a backup.

It sounds complicated, but it is not. It works simple. You just ditch the default phone app and use Groundwire.

And, here is the kicker: You can have cellular data service on two carriers for something between $15 and $50 a month, total. It’s crazy cheap.

Hint: 1) Go to your primary carrier and sign up for whatever cheap plan you want, but make sure they support eSIM and get it working with eSIM. Metro by T-Mobile is a good option, but you choose your favorite. 2) Go to and sign up with them for the other carrier you want. (Black SIM is on Verizon, White SIM is on T-Mobile.) Get dual SIM working, and turn on the “Allow Cellular Data Switching” option (which is what lets you roam between carriers.) Now, install your SIP client (such as Groundwire) and add VOIP services to it. (You can have multiple lines, multiple services, and even an office extension.)

Now you have it! Roam between T-Mobile and Verizon’s networks, WiFi calling, multiple numbers on your phone (personal, business, office extension). And, it all just works.

BTW, US Mobile is an interesting find. They are a bulk reseller and have really super cheap plans, and you can choose Verizon (Black) or T-Mobile (White). They do lack two features (that you don’t need): No WiFi calling and no eSIM. So you can only use them for one of your two selected carriers (since the other one needs to be on your eSIM.)

It really does work. Well.