Yep, this is routine practice - slow speeds, problem with your router, your direct debit has not gone through, etc etc. Usually it's a guess as to the ISP you are with, unless there's been a data breach where (for example) your landline has been associated with a VM email address. All intended to sucker people into one of the following:
1) Make a payment
2) Divulge bank details
3) Allow access to systems or data for identity fraud purposes.
The number that shows is not the source of the call. It's called "number spoofing", but the short explanation is that any number is easily programmed into any modern corporate telephony system, and the fraudsters will be operating a small call centre behind it, from locations in third world locations (the same sort of locations as big companies outsource their customer "service" to). By routing traffic through multiple countries they can make it difficult for UK authorities to find and shut them down (and even that depends on third world justice systems that are easily bought off).
The crook use UK landline numbers because they know there a good chance people will pick the call up. They can either guess a suitable number as it doesn't matter if its live or not, and in some cases they'll use the number of somebody they've previously dialled. It's also possible that they'll particularly pick on the number of somebody who, recognising a scam call, has strung them along for twenty minutes or so. They change the number frequently because otherwise it gets picked up by call blocking systems. I've even had my landline number used in this way, with the result of a few puzzled calls "I just missed your call", "I didn't call anybody", "Oh yes you did", "Oh no I didn't".
If you want to avoid these calls, the best solution is probably something like the BT 4600 or BT Advanced Phone Z, both of which have excellent call blocking or delay/deterrent features.
