Keypad not working during phone call

Question

During a phone call if I hit a number it is displayed on the screen but it doesn't seem to send it. Eg Automated voice says press 1 to continue I hit the number 1 and it shows 1 on display but then get message No input received. This is happening with voicemail and other automated systems and is really annoying. The keypad works fine for making phone calls and sms just not during phone calls. Any ideas on how to fix it ?