iPhone wont recognize simple mobile sim but recognizes different sims?

I've had my iphone 4s for about a year now and i've had it unlocked with the gpp sim and it worked fine. then 2 days ago i took out my sim to check the sim number and put it back in. and it didn't work! all it shows now is 5 bars of signal no carrier name and i cant text go on the web or call. when i call it says error number 16. my mom uses h2o wireless i put that sim in and it worked fine with no problems. i just thought the sim was dead so i bought a new one, got it activated and same thing bars no signal. i then put the sim card in a different phone and it worked. please help. thanks in advance.