Not sure about this one i have a prepaid and i have had it switch off if you go to my account on a computer for your account scroll down to Settings click Settings and it should come up in the list and see if Premium services is turned on or off.
You do know 1900 numbers if your able to call cost a lot more then normal number to call.
Unless there has been a call barring for 1900 numbers on your service it should allow it. Fore more information on calling 1900 numbers you can visit our site here
Additionally If you wish you can PM me your details and I can have this looked in to for you if you wish to call 1900 numbers.