Can you set the telephone to 'tone' to see if it fixes the issue?
Check on the back or underneath the phone for a switch that says T or P, it will need to be on T. This could sometimes be on the side of the phone and they vary depending on the make.
• If it is a cordless phone, you will need to go through the menu setting on the handset's display, look for settings, tones, key tones and set to tone dialing.
• If you still can't get the tone to work, you will need to consult the phone manual or speak with the manufacturer/shop it was bought from.
Regards