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I can call out but cant recieve calls
Have you tried resetting the phone modem? Also have you checked if call forwarding is in place?
a plug-in you've used before or an additional phone jack??
I had a continuous busy dial tone when I picked up the phone a few weeks ago. I did like WiderM said...but my separate phone modem was in a COX box in the closet. My home has phone jacks in the rooms and I have a answer machine with 6 cordless hand sets. I cleared the closet area and did a power reset of the modem inside the box. I have a separate computer modem for the PC with ethernet/wireless in another room.
Also log on to your COX phone account and view your Call History. You could see lots of Phone stored messages that went to the COX voice mail instead of your phones answering service. Also, like bearone2 says, try another phone into the jack...maybe its the phone. Note: if you have the COX phone system, I remember there is some setup switch toggles that controls when your Cox Voice mail kicks in vs. how long your basic phones rings before it answers...maybe you have some numbers blocked with no ringer...again, the Call History could give you an insight to who called you when it did not ring.
I had a similar issue a while back. When I made a call and got a busy signal, the only way to clear the busy signal and get a dial tone again was to power off the modem. But, it's fixed now.