First off this is absolutely a scam and don’t ever ring them back. it will be a premium rate phone number or they will answer and start to ask you to perform some actions on your PC to give them access to your data.
First thing is to ensure you are registered with the telephone Preference Service
This can take a while to filter through so you may still get some calls in the interim and it can’t stop all calls from abroad.
There is a lot more info on phone scams, how to recognise them and what to do about them on the AgeUK website
If it continues to be a problem for you then you should contact your telephone provider and/or your internet provider. They have a duty to prevent spam and scam.
Something else you can try is to have caller ID and screen them out that way but you can also look phone numbers up on the internet by going to a site like WhoCallsMe. there you input the phone number you want to check and it will tell who it is and if there are any reports of it being a scam etc.
My Mum has a call screening on her line and it won’t allow any call to get through unless it recognises the number. If it doesn’t recognise the number then it asks the caller to leave their name and to wait while it contacts her, it will then ring her landline, ask if she wants to take a call from the recorded name and then puts it through or dumps it accordingly. She was at one point getting 20-30 nuisance callers per day, now she gets none.
Hope this helps.