I think theres something wrong with my simcard?

I got a new phone but used a simcard i already had. When i put the simcard in the phone, and tried to text the messages wouldnt go through. when i called the phone company they said that since the phone was a pda that you needed a certian simcard to text. then i got another phone [my moms old phone that was used for testing when her simcard was in it] and tried to text but the messages still dint go through. what does this mean? please help me!!