There are only one settings you can change on the phone itself which may fix it.
Changing pulse dialling to DTMF dialling will send a tone rather than a pulse to the exchange, however by default your phone should already be using DTMF.
To check this, press the menu button > Select Telephony > Select Dialling Mode > Then chose DTMF.
If the phone is already set up as DTMF, and your standing in range of the base station, I am afraid it time for a new phone.
Yes, the tone setting is DTMF, which is the standard (modern) mechanism for sending key presses to the other parties exchange.
If your not having issues with signal\sound quality, it is most likely to be an issue with the phone processor not generating the tone at the right frequency, with alot of customers going digital there is less tolerance, and probably why its working intermittently.
I am sorry I couldnt offer a fix for your issue.
Have a good day, should you have any further queries please do not hesitate to contact us again.