not sure if this a software issue or phone issue

[mikeeemm]

I have a Samsung focus sgh-i917, when im at work (inside mall) in one of the 2 stores I don't get any reception, sometimes I will get reception when i enter other store or a bit after or if i restart my phone, but if I stay in the store where I dont get reception it takes me 15 minutes of driving home to get reception. What causes that? If I restart my phone outside of store I get reception