I am purchases a smart watch but I have a phone with service. Do I need a sim card for the watch? If so will I have to pay a fee for service every month or can it just hook up to my phone?

I purchased a Samsung smartwatch for my daughter's Samsung S7 smartphone. Did not need anything extra. Set up was soooo easy. AT&T. FYI

The watch needs a Sim card. You need not pay extra since this is not an additional provision. Just an extra Sim, same number, so billing remains same. Need to seek permission for using two Sims, it's easy.