Can you pleaase help me with my phone?

Well, about 2 weeks ago, I bought a Blackberry Curve 9300 3G in Fuchsia. I bought an Orange 3G sim-card and it gave me a text to pay �5 a month for BBM, unlimited internet and social networking. I was going to reply but when my friend told me T-Mobile give 6 months free BBM and 6 months free unlimited internet I thought of changing to T-Mobile. When I had gone to the shop I bought a T-Mobile sim-card and a case, for �6 altogether. When I had gone home I tried to send a text to my sister and it said it was unavailable. I thought maybe it didn't Top-Up properly so I phoned T-Mobile from my phone.(as I have it free) Then it said 'Unavalible' again. I looked in the corner and saw it saying 'SOS' I don't know what it means but I think it means no signal. I don't know what to do now...Is it my phone cant take a sim-card??? (my phone is unlocked)