When using a headset with an RCA 2524RE-1 phone (and this may also apply to model numbers close to that one as they are from the same series) and the phone screen says INHIBITED, here’s what you can try:
1) Unplug the phone line from the wall.
2) Push the phone cords all the way into the phone.
3) Plug the cords back in.
If you are still getting the INHIBITED message after that...
Make sure your phone is not plugged into the handset jack but trying to use the headset button on the phone.
If you are set up this way, then you’ll need a 2.5mm adaptor for wireless or QD cord for corded so that you can go through the proper headset jack.