This is most likely because you haven't got Flexcredits on your account or a Monthly Plan enabled. SIP calls are chargeable, so you need some method of funding them.
Also, if you try to make calls with your display name set to a number or name that isn't one of the Flextel numbers in your account, then the call will be rejected.
The final reason calls may not work is if your softphone is configured with an unsupported codec. You need to ensure that the softphone uses GSM, G711 (aLaw or uLaw).