Checking number on Nokia 6100 & O2 credit?

[stockhausen]

I have an incredibly old Nokia 6100, lacking a back cover and with illegible 2, 5, 8 & 0 keys (but it still seems to work) in which I have an old O2 PAYG SIM card.

Does anyone know how I can interrogate it to find out what the 'phone number for the SIM card is and how I can check whether there is still any O2 credit on the card?

Thanks

 

[rscosworth]

*#100# ususally for the number

 

[stockhausen]

*#100# ususally for the number

OK, many thanks for that.

I am now unsurprisingly getting the message "Inactive SIM" - does anyone have any idea how I can get this SIM reactivated and how frequently I have to use a SIM card in order to keep it "active".