well i am not an expert on this issue by any means,
first i would have to ask how the signal is in the area your are working with,
second i would have to ask if when you setup the phone, if you did it as it said on the activation card,
Third thing is a slight possibility you got a bad phone and will have to exchange it at your local walmart for a new one,
(it has happened we just got my daughter one Thursday night) and had to take it back 3 times before we got a one. (hers was unable to charge unsupported battery)
there are many things that can be the problem here with yours.
customer support at Straight Talk isn't much help for anything. i know i have been using their service from almost a yr now, the service is good when its working right, but if you have an issue Customer Service is worthless for the most part.
Keep this in mind when having a problem,