Fix "Mobile network not available error"

[darkewatters]

I apologize if this is an annoyingly common question, but I have been scrolling through the site for a while and haven't found what I'm looking for. I just bought a Samsung Galaxy S from a friend and have managed to set everything up but the most important part- calls and texts. I have t-mobile so I put in my SIM card and used the factory reset option to clear all previous data. I got it hooked up to my email, updated my apps (both the 4g and wi-fi work great), and all my contacts synced but when I try to make calls I get the error "Mobile Network Not Available"

Help!