I believe you mean Spectrum Mobile.

And Spectrum Mobile service is actually provided by Verizon, they don't have their own service, it's a repackaging type of deal.

So AFAIK, having Spectrum Mobile is the same as having Verizon in almost all meaningful ways.

That said if you're meaning to keep using your existing AT&T phone, good chance it's not going to work and you'll need new phone(s).