Travel Sim

[Przemolog]

Originally Posted by Bossman

AFAIK, all phones have a call waiting menu where you can activate, deactivate, check status of call waiting for all types of calls, voice, data, fax(depending on phone).

Yes, you're right. Phones have menus for managing call waiting as well as call diverting/forwarding, barring, enabling/disablling caller ID. There are also underlying GSM network codes composed of */#/digits (e.g. *43# for call waitng activation as Keighleyboy mentioned) which do the same.
But in the real world there are also the following possbilities for managing the services in question:
1) Non-standard operator-dependent codes (USSD)
2) Voice prompts + DTMF
3) Web-accessible user accounts
4) Call/e-mail to Customer Support

It's also possible that it's the operator policy not to let users enable/disable services they need at all :P .