If the other phones had strong signals where you are, it may be the phone. If you're uncertain, return it. Remember though, to return it for free, it must be within 1 month of the contract date (which you know) AND you have to have used less than 1 hour of talk/text time. The second part is important, since we all know how fast that goes.
if you are past the 30days/60mins of use, you still MAY be able to exchange the phone under your 1yr warranty, but the bad part is that it will be exchanged for a "reconstructed" (refurbished) phone...

it might fix your problem, but its up to you if you're ok with trading it in for a different phone..
Oh, and if you are still within 30 days itll be free of charge, $10+tax after that.