tell him to dial the number with a # at the end as this will cause the phone to send the digits to the pbx
or edit the basefile to change the timeout value - i am not 100% sure (perhap the sangoma gang can comment) but i think the parameter you might be looking for is
entryTimeout # NO Key Entry Timeout(in seconds, 0 means never timeout, default is 0 seconds)
change the value to be say 3 seconds. what i think this should do is cause the phone to wait 3 seconds after a digit is entered and if nothing else is entered it will send the number to the pbx. the only side effect of this is that if you have someone that is a very slow dialer (more than 3 seconds between digits) the phone will initiate the call with an incomplete number