Modern BT exchanges here in England reject them and go to the standard (and irritating) "The number you have dialed has not been recognized" recording. It's not always an immediate dump to the recording; it depends upon the position.
The BT automated dial/keypad test has an option for them here (for UK readers, this is the line tester reached by dialing 175). After the line test and ringback when the system prompts you to "Dial next test," the normal method for checking a DTMF keypad is to dial a 1, then when you get "Start test" followed by a dialtone, you dial
123456789*0#.If, however, at the "dial next test" prompt you dial A, you will get the same "Start test" folowed by a dialtone, but the tester then expects to receive just the four extra TouchTone digits ABCD in sequence.
Paul Coxwell Eccles on Sea Norfolk, England.