Roaming, voicemail and diverts question

Hi, could anyone provide any answers to the following question, as T-Mobile's site is not clear on this...

When roaming, rejecting a call will normally incur charges for receving voicemail, and the normal advice is to divert all calls to voicemail.

However, does anyone know what will happen if all diverts (busy, unavailable and unanswered) are turned off completely? Will it still give an engaged tone for the caller if the call is rejected/phone busy when roaming? Can they still charge you anyway if this happens?