Yes, the HTC Diamond will work for T-Mobile unless it is locked with any other network.If it is, then you need to unlock it.To unlock an iPhone remote unlocking method is the best method. You can get the unlocking service from network provider, sometimes they deny. In that case you can get the remote unlocking service from trusted third party like Theunlockarena.com and follow the free unlocking instructions to easily unlock from the locked network.They unlock various mobile models.