Newbie needs advice on which sim card to put in GSM Modem

Hi guys.

I am new here so feel free to tell me if Iam in the wrong forum or section.

I have a Cinterion (Seimens) GSM Modem and I want to sent text messages with it.

I have written software for the operation, and I have tested it on a Dell Laptop with a built in modem. all good.

However, I need a sim card for the modem and that’s where I get stuck.

I have a collection of errors like sim not inserted or the message sends but is not delivered.

Can anyone tell me what sim card supplier/tariff I need to get?

The cards that I have been testing with are sending ok from my old Nokia moby so I am sure that they can send/receive in a normal environment.

I have tied just using hyperterminal and it fails to send as well, so I know it’s not my software.