I have a NEC DT300 WIRELESS PHONE and the speaker button will not switch on and the mic does not work also it will not ring it just hangs up i am hoping that all are conected to each other

SOURCE: change the time on our NEC DT300 Series

What model of NEC phone system do you have? Go look in the phone or data room and look at the PBX. You might also be able to press Feature 3 to get some information which would be helpful to a tech. If it is an SV8100 try this:

To set the system Time:

1. Press

Speaker.

2. Dial

728.

3. Dial two digits for the hour (24 hour clock, 13 = 1:00 PM).

4. Dial two digits for the minutes (00~59).
5. Press

Speaker to hang up.

SOURCE: Call forwarding

As Kakima said, it's with your phone provider. If you have trouble with that, you can always set your voicemail to a temporary one telling the people who call you to call your cell phone number. Good Luck!
-V