Any Phone Gurus

i finally decided to get a dang phone ....im sick of never being able to call ppl during the day when none of my family is around.

i am trying a T-mobile SIM card from Wal-mart ....one of those pre-paid units.

i got the sim only bcoz i already have a phone that didnt have a SIM card

so first problem is i had to shave the sim card down to fit in the Micro sim slot.

and it works getting and recieving calls but only for T-mobile numbers ...all the other numbers say "no mobile network available" i been messing with this all day!

the phone is an unlocked Galaxy S3....it originally came with a Saudi Telecom sim card bcoz the phone was bought new in Saudi Arabia...but its an unlocked International version ....so it should work with any Sim on the planet.

is there a phone setting i can use to fix this problem?