Never pick up random calls. Ever.
For various reasons.
First the obvious one: You could be paying (or whoever's paying your phone bills) for receiving calls from these blokes, or for calling them.
That, and you don't know who they are. There are scams when a random number is calling your phone. Either you see the number and don't recognise it, or it's shown as unknown. When you try to pick up, it could be they just hang up. The calls can be made from anywhere (even a satellite phone) and they can actually make you pay a lot of money for that joke, which the scammers eventually receive. (Complicated process to explain, but the main point is: Don't pick up random numbers.)
It could be the new phone you got either has an older re-used phone number, or someone has been selling your contact information through to companies, other third parties etc. (If you have a new or old phone number, this could happen.)
You should just be able to block the numbers. I would go ahead and do so. Keep the numbers, just in case, but make sure you can't accidentally call them or whatever.
