There was a law passed a few years back (on a Federal level) that required phone companies to allow customers to keep their (mobile) phone numbers, regardless of switching between providers.
So the short answer is YES, YOU CAN KEEP YOU’RE PHONE NUMBER
The slightly longer answer is that there is a small caveat that I’ve come across when doing a phone number port on my own:
Essentially, you can keep you’re phone number, but only within the same area code meaning that if you have a NYC phone number, and move to LA, you can’t have an LA billing address and still have a NYC area code phone number. The law technically reads that they can only port numbers within a single area code.
The easy way around this (if you want to keep your phone number, and switch carriers) is to use some alternate billing address within the other area code that you want to keep.
This recently happened to me, because I lived in DC, but had a WA area code.. so when i went to switch services and port my number.. I had to set my billing address as someone I knew back in WA, so that the new service provider could then give me a temporary WA number.. and then port my real number over to that temporary WA numbered phone.