Why does credit run out on payg without it being used?

[Genny]

Hope someone can help. I have a phone I almost never use. I'm a pensioner. I use the mobile for emergency calls when I'm out, and for receiving calls at anytime. At home, I use Whatsapp to make calls. This phone has an O2 PAYG SIM. I have three questions:

1. If I top-up, say with £10, will I lose that credit if I don't use it to make phone calls? (I topped up on Dec 15th, yet again with concern that the money would be lost. I made three UK calls of 3m54s, 9s and 13s since then.)

2. Is it at all possible to see the cost of any mobile calls I do make from the top up amount, online?

3. Could spam calls, if I answer them (and get no response), be taking my PAYG credit?

[MI5 · Accepted solution]

@Genny 

All explained here https://www.o2.co.uk/help/pay-as-you-go/big-bundles

Basically when you add credit, it is exchanged for allowances that last for 30 days.

There is no online way to see what you have used with a PAYG account. You'd need to call O2 on 4445 to find out that info.

Answer to Q3 is no.