I pond-hop frequently USA/UK, have done for > 20 years so I keep USA and UK phone numbers.
I have in effect 7 phone numbers. I have in USA ATT, T-Mobile, an ATT MVNO and Google Voice and in UK 2 Sipgate numbers and a O2 MVNO (Giffgaff) which ring a SIP VOIP app.
I only give out the GV number and program it to call the appropriate real number to match the device in my hand, whether its my smartwatch, phone, tablet, laptop, etc. You can not really know where I am physically other than audio lag if its going over Atlantic.
ATT is currently receiving the most robo calls. Never get any robo calls within UK. T-Mobile used to be bad but is better now.
There are various guides if you want to use the address book method instead. I tried it, it did work but I need my phone for work and blocking people who I didn't know was just leaving an horrible 1st impression.
I don't know how Google Voice is doing such a good job but I wish ATT would copy their method.
The caller is usually the same first 6 digits as my number and random last 4 digits.
Now I think of it, what I could do is only accept calls from Google Hangouts and block all phone calls but that mean all calls are VOIP?
I think this is a scourge and wish the telcos would do a better job of blocking them. It must be obvious to them its a robo call as the caller is randomly ringing around and it would only take a few successful calls for the telco to know its a scammer? Google is able to do it, why not the telco?