Previously, you could only use an iPhone on T-Mobile if it was an older model that supported the normal sized SIM card. In order to use the most recent iPhone, the iPhone 4, T-Mobile customers needed to use a micro SIM card, which T-Mobile did not offer at the time. Today, T-Mobile began selling the micro SIM card for use on any GSM (locked or unlocked) or iPhone 4 (unlocked) phone on T-Mobile’s network. While the micro SIM will allow you to use your iPhone 4 on T-Mobile, we must warn you that you will only experience 2G EDGE speeds, instead of operating on T-Mobile’s 3G network. The micro SIM cards retail for $9.99, but are currently available for free online.