Try a factory reset as per the following:
Step 1 
Unplug the power cable from the phone and then plug it back in.
The phone begins its power up cycle.
Step 2 While the phone is powering up, and before the Speaker button flashes on and off,
press and hold #.
Continue to hold # until each line button flashes on and off in sequence in amber.
Step 3 Release # and press 123456789*0#.
You can press a key twice in a row, but if you press the keys out of sequence, the factory reset will not take place.
After you press these keys, the line buttons on the phone flash red and the phone goes through the factory reset process.
Do not power down the phone until it completes the factory reset process and the main screen appears.
Let us know if it works.
Manish
