My phone says line in use and no dial tone

SOURCE: no dial tone and "line in use " message

Try the simple things first. Try a different phone. In my case, I had the phone company check - outside was fine. After two hours my phone worked. I figured out what happened in my case. Both phones were dead (battery). I think one died with the phone off the hook. once the phones were fully charged, the phone worked.

SOURCE: intercom light stuck on and no dial tone on GE 4 line phone

I bet the problem is that you used to have two phone lines and now you only have one line. You cannot program an extension into one of these phones unless both Line1 and Line2 coming into the phone are working- it will act as if the extension you picked is in use, but the actual thing it is complaining about is that Line2 isn't working. The phones might have been working for years on a single phone line as long as they didn't lose power. But you don't have to throw the phones away just because you gave up your second phone line, you can still make them work. (I did it for a 12 phone system.) The way to to it: make a special cable that connects your single incoming phone line to both L1 and L2 on the phone, tricking the phone into thinking both L1 and L2 are working. Then program the extensions into the phones, and go back to using a normal cable so that only Line1 is hooked up. To make this cable: take a normal male to male cable and cut off one end. Then connect both the black and the green wires of the cut end of your cable to the green post in a standard phone jack (the kind you would surface mount on the wall will work best), and connect both the yellow and red wires to the red post inside the surface mount phone jack. Now you have an "extension cable" that takes your one incoming line to both L1 and L2 on the phone: Now plug your incoming phone wire into the phone jack, and plug the wire you connected into the jack into the back of your phone. The phone is probably already blinking red on the intercom, just push the extension number you want it to have, making sure there are no conflicting extensions already in use.
I would reconnect the phone using a normal line so that only Line 1 is "hot", don't leave it on this double-connected cable any longer than you need to in order to program the extension.
Good luck!

SOURCE: Phone line/phone jack problem

The wiring on the jacks is set up for a 2 line phone.

If you take the jacks apart, you will see either:
A) red, green, and black, yellow
B)white blue/blue white and white orange/orange white

You need to swap the pairs.
If there are red/green/black/yellow wires. remove the yellow/black and move them to where the red/greens are. Then put the red/green wires, where the yellow/black used to be.

OR, you need to swap the orange and the blue wires.