We need to setup a voicemail for our business DT300 NEC phone that will then divert the caller to the relevant dept eg to speak to a pharmacist press 1, to speak to a Beauty advisor Press 2 etc.

SOURCE: Keypad not working during phone call

If sounds like DTMF is turned off on your phone. If it is then it is not sending the correct tone over the air which is why nothing happens. Go into setting, then to initial setup and key down toward the bottom and you should see DTMF if it is off change it to short or long and that should correct you problem.

SOURCE: All my Calls go Straight to Voicemail

Here is a link to the user manual for you
http://www142.nortelnetworks.com/bvdoc/norstarbcm/t7316.pdf

SOURCE: calls go straight to voicemail

You please change the setting from answer voicemail to ring mode if you have any dificaltey refer your phone user manual book and reset the option

SOURCE: iphone can't receive calls

check in settings/phone to see if you have maybe turned on call forwarding and it is being forwarded automatic to your voicemail box number

SOURCE: number of rings before going to voicemail on KX-T7630

Press the FWD/DND button, then scroll through the menu button until the screen shows "FWD No Answer time". Then use the number key pad to change from 000 seconds to 010 seconds (or longer if required). Then exit out of the menu.