No, you do NOT have to give up service.  Using XMPP for VoIP is the number you’d communicate with.  This can be TOTALLY DIFFERENT than the number you get literal service from.

For legal purpose I’m really in international waters,
But let’s say hypothetically I’m standing in Texas with a Texas SIM card getting service on my iPhone.
I can then use XMPP to text or call with a Miami Florida NUMBER, even though I’m nowhere near there.  If you don’t use the number you’re getting service from, you’re hiding your location.