Spammers use fake phone numbers all the time on the regular phone service

The phone calls just use fake caller ID. Caller ID is entirely unauthenticated and the recipient just blindly trusts the sender, so scammers use sketchy VoIP services that let you override the caller ID without actually proving you own that number. Work is being done to improve this: https://www.fcc.gov/call-authentication

That's means it's trivial to use a fake number for outgoing calls, but the spammers can't actually receive incoming calls or texts to those numbers.