Have you got, or can you borrow, a multimeter?
The phone line should be 50 volts. I very much doubt it will be lower.
Edit: as @RoyB says, the only way to find out will be to measure it.
If you have only just bought the Panasonic, did the caller ID work on the previous one, or did it not have the function?
Interestingly (well I found it interesting), we were discussing the line voltage and caller display on the Vodafone forum recently. It actually started out about digital voice and the adapter needed, but quickly went off topic.