I don't get it. If it has it's own sim card it will have it's own phone number? that won't work. yeah it will be nice to be able to make phone calls without phone, but receiving them is equally as important.
For google voice users I guess they can add the second phone number and be covered, but not every one has google voice.
Majority people will always carry their phone with them. this will make a good running watch if it can stand up to constant sweat. you can run without your phone and still have a phone for an emergency and listen to tunes directly from the phone.
from what the samsung reps and carrier reps have told me, at least on at&t and tmobile, since they are GSM, the phone number will be mirrored on your wearable, so that you won't have two different phone numbers for calls and texting. Haven't heard if that is the case for sprint or verizon though ( CDMA ).