I'm trying to place a voice call using a GSM mobile phone (Philips
Fisio 825). Therefore I'm sending ATDT <number>; and the call gets
initiated as expected.

The only problem is that now I would like to to terminate the call
after a fixed amount of time has elapsed (say 20 seconds) and I just
can't seem to get it right: ATH only works after the callee has
answered the call and even closing the serial com port doesn't help
(phone keeps on ringing).

I know that with data calls I can use ATS7=x to specify the # of
seconds after which the call should be terminated if no connection is
established, but what about voice calls?

Any help would be highly appreciated.

Wilson