Hello Horst & Welcome to Just Answer,
Sorry to hear of the issue you are having with your phones.
To help you with this issue, may I ask:1) WHERE these handsets were purchased and are they NEW or REFURBISHED (used)?2) How many TOTAL handsets were EVER in use with this system?
I only ask because it is true that there can be up to six handsets registered to a single base unit at one time. But if there were more registered in the past, those handset "slots" in the base unit may still be marked as "in use".
May I ask what "number" the working handset registered as (1-6)?
Also, what happened when you tried the others (what did you see on the displays)?
I see it has been a few days and I just wanted to follow up to see if you have any more questions concerning this issue or if the issue was resolved.
I look forward to any further questions or results that you may have.
Thanks, ***** *****--. .-(im)