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The first thing to try would be, with the phone in this bad a state, just powering the phone Off, and unplugging it from the phone line. Leaving it that way for a few minutes or longer, then powering it On again, with no phone line connected, and see if that cures the Line In Use indication, at least.
Listen on the handset for any tone or message, also, at that time.
Then let me know what happened, and how old the VTech phone set is, please.
Is the Line In Use light on the handsets, or on the base station part of the phone set?
If the phones are being deregistered, then you might need to re-register them to the base station. Does the manual you have show how to do that? if so, I advise trying a Register Handset procedure on each handset.
Let me know how that goes, and if you need instructions, etc.
(I regret that I am not able to offer Live Phone Call service at this time; However, your request is still open to other Experts in this category, and one of them may pick up on it at any time.)
OK. Let me know what the result of the registering - or, re-registering - procedure is, please, and how it turns out. I'd like to see this case get to a satisfactory result.