Is stealing a phone and reconnecting it after it is disconnected Identity Theft?

It's identity theft if she pretended to be someone else to obtain services in that person's name. That may have occurred with the Social Security number, but it's not clear how she would have had it. If she did have it, she presumably obtained it via your son.

More than anything, it sounds like it's theft of the phone.

It's not really something you need to solve one way or the other yourself. If you'd like the phone back, you can contact the police on their non-emergency line and request their assistance. You can mention her suspected use of your Social Security number in the process. They may or may not pursue it further, including questioning your son about your Social Security number.

Reconnecting the phone is NOT ID theft. It is merely old fashion theft. Using YOUR SSN is ID theft.