I have kept my iPhone 2G from 2007 and it's still fully functional. It was O2 only at the time so I assume it will not work on other networks, please tell me if I am wrong. I was wondering if there's still a SIM only tariff that I could to get the phone online for calls only?
Yes it will be locked to O2 only and unlikely to work on any other network, however, any of the tariffs sold today would still work for voice calls.
I still have current PAYG sim in my Motorola StarTac from around 1998 ish.