I purchased an E61 from Nokia and used it quite happily with Virgin.
When I saw T-mobile advertising its SIM cards for 50p I thought I'd
give them a go. Bad choice.
Right from the start there were problems. Their website wouldn't
recognise my SIM and despite clearly showing its phone number at the
top of the page tried to make out it didn't exist and that I was
merely a 'guest'. There followed several weeks of email toing and
froing with customer support, all to no avail but I did manage finally
to top up the SIM online. Another bad choice. A few more weeks elapsed
during which the phone was virtually unusable on the T-mobile SIM,
frequently crashing and generally misbehaving with endless popups
about the errant SIM but was still fine on the Virgin SIM. The last
email I had from customer services assured me that the tech team had
been informed and that the website would be put right but nothing has
happened. Finally, after much hassle I have had to admit defeat and
abandon all hope of ever using T-mobile on my phone.