You can use any phone carrier you want, one of these two real ones, or one of the hundreds of VoIP providers. Regardless on your choice, you can still call the

You can use any phone carrier you want, one of these two real ones, or one of the hundreds of VoIP providers. Regardless on your choice, you can still call the people you want, regardless on their carrier. The only difference is cost per minute, which is typically low cost to switch. It's impossible to use an alternative messenger to communicate with people who use FB messenger. Unlike e-mail providers or phone operators, these newer web services don't have any interoperability, they're closed systems. To switch from Facebook, a person gonna need to convince all people they communicate with to switch as well - to the same alternative messenger. That's very high cost to switch, borderline impossible to do for many people.