Hello & Welcome,
I'm Greg and will do my best to assist you today.
Sorry to hear of any issue you are having with your phones.
To help you with this issue, may I ask:
1) The FULL Model Number of the phone system you are working with? If this is a cordless system, that model should be printed on a sticker on the bottom or back of the MAIN base and START with KX-TG.
2) How many TOTAL cordless handsets do you have with this system?
3) What is the AGE of this system and all of the handsets that came with it?
4) Is this one handset good for RECEIVING calls?
If you can, please CHECK if the handset can take calls.
Just pick up that handset and press the TALK button, and listen for the dial tone (let me know if you can hear that).
OK - is there ANYTHING on the display of that one handset?
Also, have you EVER replaced the batteries in ANY of the handsets?
OK - please tell me EXACTLY what is on the display of this particular handset.
OK - the first thing I would do is try to RESET this handset.
You MAY be able to do that by removing the batteries from this handset and leave them out for AT LEAST 15 minutes. The put the batteries back and and then TEST for a dial tone - as we did before.
If that does not restore function, then please SWAP the batteries with the handset that has the NEWEST batteries, and see what happens when that is done. Also note if the issue "follows" the batteries.
Please give that a try and let me know your results.