I was sent a new SIM card ten days ago for my ancient 2006 PAYG Nokia 1100, and have been putting off swapping it, having read so many tales of woe on here.
This morning, I finally bit the bullet and guess what - it just worked!
On powering up the phone, it asked for the time and date, and shortly after, a text message arrived to say it was working. No unlock codes, no fuss. Where it used to say Virgin on the home page, it now says T-Mobile UK.
I bought the phone from Carphone Warehouse in 2006 and can only assume it was sold unlocked. I managed to find the original box and receipt and fwiw the stock label and shop product code had nothing to suggest the phone was tied to Virgin.
Anyway, just thought I'd share a bit of good news, along with the info that unlock issues aside, an ancient PAYG Nokia 1100 can still work with the new SIM.
Just to add - the phone was bought along with a Virgin SIM, which I think was fitted in the Carphone Warehouse shop at time of purchase. I think the Nokia 1100 was available on a few networks, I chose Virgin as the SIM card came with a 'free' £5 airtime voucher!