Cordless phone behaving abnormally - any ideas? (It's probably something really simple!)

I know I could probably replace it easily enough, but in our fairly remote area it’s quicker to try and fix things (and we need our phone!).

It’s a Panasonic KX-TG6811E (with 2 handsets), though users of their other models or makes could well have the answer.  It may simply be down to some feature we’ve never used, but invoked unintentionally (e.g by trying some button to see what it does).

It’s started ringing, very loudly, at regular (I think) intervals of perhaps 10 or 15 minutes.  (Quite like its usual steady one-note rings when someone calls, and with the same rhythm I think, but probably loud enough for a disco).  So far, the only way we can shut it up is to leave the mains power disconnected from the base unit.

It’s been a bit confusing to diagnose, as it started directly our power was restored after storm Eowyn.  We did have some strange bits of electrical stuff happening during the night of peak storm here (including with this phone), but it seemed unscathed as usual.  (Perhaps it is).  The displays on the handsets were as normal, I got the usual dialling tone, and the handsets and my mobile were able to ring each other (only just, but that’s down to our extremely weak mobile signal).  Our area and house have always attracted lightning, but our Panasonic has sailed happily through storms throughout the 10 years we’ve had it, including plenty of direct or near direct hits to the house, and we’ve always left it on 24/7.  (It’s been the same both before and after ditching our old Eircom line for a Voip service).

I haven’t yet timed the frequency of these cacophonies, or the number of rings each time.  (That needs a time when nobody’s trying to concentrate, listen to something, relax or sleep!). But from memory), at a guess I’d say it rings for about 30 seconds per session.

From a look through the instructions, it seems that the culprit could be one of several features; Paging, or the Handset Finder (or maybe those are different terms for the same thing, depending on which section of the instructions they’re in?)  Or an ‘alarm clock’ facility, or a distress call.  (It certainly seems penetrating enough for any of those!).  If so, it’s hard to know which one, with not knowing what they’re supposed to sound like.  The only mention I could find for turning any of those off was re Paging (press the big button on the front of the base unit, and the red ‘End call’ button on the handset(s); separately, or at the same time, wasn’t specified.  (That didn’t do the trick, and was quite a physical challemge to do, so I’d guess it’s for paging an extension, or another person in the same building, rather than finding a handset?)

If the cause could be surge damage, I could easily test that with an unused spare set of the same model (base unit & I handset), that I bought at the same time, meaning to put a 3rd handset somewhere   I’m not sure about the pair of handset batteries that came with it (and rechargeable batteries are unavailable locally) but we happen to have 2 pairs of the correct type in the house, that I bought just a year or so ago.