Don't take it too seriously. Talk with them.
I had a random prank call from a girl once - I remained calm and polite, kept a sense of humour and went along with it giving as good as I got when needed. She told me her name and location after a few calls and we ended up chatting a few times, it was quite fun - then it eventually fizzled out.
Because of what you hear happening to people that meet people and talk over the internet I don't think that is very good advice to give anyone even though it is a phone call. Sorry but I would just ignore any numbers I don't know. Just airing my view on the subject.
You could always try and put the numbers on to www.phonespamfilter.co.uk and it may be able to tell you who is calling you. This is particularly true if they are a large company or a sales dept trying to flog you stuff
If you think it from a business, type the number into google or google for 'who calls me'. There's a forum where people list unknown numbers and identify them if possible. I've dodged many sales calls by checking on there first.
FYI - if they are sales calls (or anyone suffering from them) you can sign up to TPS (they are a regulator) who can remove your number from sales companies.
(once a sale company has your number they can sell it on to others!)
do 1471 if call number is hidden ring your sevice provider and ask them they may have the number,new laws coming in their is a 2 million £ fine if you can identify the number to stop cold calls .
be careful wo you phone back th is a scam where when you call back you are hooked up to a 50p per minute call and you cant disconnect for about 20mins.
If the caller has with held their number its usual a call to try and sell you something, yesterday I got a "congratulations you have won a holiday in Florida" call which I politely told the to pee off then realised I was talking to a recording !
If you dont recognise the number dont call back, if its important they will call again.