My Cisco 7940 phone can receive external calls but can't dial out. It can only dial local extensions in the building.

If there are lines through your line appearances than your phone is likely not properly registered with your Callmanager or SIP registrar, you need to configure your phone properly so it registers and associates with an extension on your PBX or hosted service.

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get a dial tone and dial 7091.

do the same with 7090 to turn the message light on.

can you explain me how is your phone connect to internet.
best on your Question its seems to me you cant hear a dial tone because of following one or more factors might apply

1. you must log into the Extension Mobility service
2. you must enter a client matter code (CMC) or forced authorization code (FAC) after dialing a number
3. your phone has time -of-day restrictions that prevent you from using some features during certain hours of the day

You would need either a call manager or call manager express to configure the phone and connect to the phone line.

The handsets will eventually go bad. If it is every single handset that you have, the problem is with the system. But if the problem is only with a few of your phones, you might consider replacing the handsets. We at IdacomUSA.com have replacement Cisco handsets for sale at only $25. Hope this helps you out :)

1-800-903-9190

It sounds like you have enabled Do No Disturb accidently. This can be turned on and off by pressing the DND softkey button along the bottom of the LCD display. You might need to press the 'more' softkey to display all options.

Give me your number, and i'll call you! Solved! =-D

Try adding Cisco 7914 (time two) for additional (other) phone lines, or define caller group so phone call rings around.

No most system are maxed on the phone ... but if you had a panasonic system they are know for multi conferencing.

Sounds like a firewall or ACL.
while offhook, press the "?" 2 x, see if there the phone is sending packets or go to the network settings and get the IP Address.
You could try to reset the phone to factory.
http://cisco.com/en/US/products/hw/phones/ps379/products_tech_note09186a00800941bb.shtml