Straight talk phone from Walmart, help please?

It says I have 4 bars of service at my house, yet it won't let me send texts or make calls (has a picture of a phone with a red line through it next to my signal). I put the zipcode of a nearby town instead of my own, but it's only 15 minutes away. If I have to change my zipcode, can I do that online or do I have to call customer service (Id rather not)