Hi
If you are on Safcom divert your calls to Voice Mail and after the person calls and the Voice Mail server indicated you have a new voice mail message, dial into voice mail and if it is the safaricom Voice mail menu hit "8" and it will read ut for you the caller ID. I haven't used the airtel voice mail yet but it should work the same way.
But after identifying the person .. don't challenge them yet .. have a lil fun on yr own ... hit them back in the pocket ... by ..
... record a pick up greeting or voice mail greeting that sounds exactly the way a caller hears when the he/she calls a number. Configure to play it back when a call is picked or when it goes to voicemail. This way you can pick the call or it goes to voice mail the person only hears or continues to hear the phone ringing ... at the other end ..
Moral of the story ... ... since the call is ACTIVE .. it will be depleting his airtime while he/she is still thinking that you have not yet picked ... 3 or 4 loooooooong calls after that you will see him/her complaining on this list that GSM provider so and so charges even when calls are not picked...