Yes, you can use your existing T-Mobile SIM card in this phone, just discard the pre-paid SIM that came with the phone and install your existing T-Mobile SIM and off you go - doesn't matter what rate plan you have. I had a talk only plan and used this phone (no data plan), and used the wi-fi at home and any place I went and surfed the internet. I switched over to a $30 talk/data plan and couldn't be more happy. T-Mobile is a good company, and I've never had any customer service issues with them - been with them for 9 years. If fact, they helped me set up an AT&T branded phone a few years back to allow all the functions to work, they didn't care it wasn't their phone. So glad the AT&T purchase of T-Mobile didn't go through!!
You get a free sim card with this which has a $ 4 balance on it. and moving your number from one sim to another is free. Why do you want to throw away $ 4 unless you want to get it unlocked by paying may be $20 and use on some other network.
man you should go connect this phone with Solavei/T-mobile service no contract $49 unlimited talk text data 4G nationwide and you can make extra $ by sharing or even free bill. I connected my galaxy note international with it man loved it no issues at all but go check out www.pay4lesswireless.com they distribute Solavie phone service no contract to.. hope this helps anybody..