Help HTC sensation foerign sim card question help please?

[alicia17]

I have a verizon HTC sensation 4g but it does not have a sim card. I use at&t and was attempting to un lock with sim unlock code but when i put in my at&t sim card it says unknown network and doesnt prompt for unlock code. I dial the number to take me to the smsc number and it is blank. I put refresh and the at&T code comes up and it recognizes the network. I have full bars and can make calls and recieve sms but when i try to send an sms it disconnects from my network completely, anybody know how to fix this or have any ideas?!