I tried this on a nokia 3410 which was locked to virgin and t-mobile sim cards were not accepted until I unlocked it at
www.trycktill.com (Make sure u change the language of the site to English unless you can speak fluent Danish lol)However, i tried another phone which was locked to T-Mobile and took virgin sims.O2 locked phones take Tesco sims.But T-Mobile are rising cost of calls next week,so i'd stick with virgin.And another thing, I once bought the 5 day mates rates pass and I was able to make unlimited calls to Mobile World customers but not virgin customers. Isn't it supposed to be to just T-Mobile customers.