Can I unlock a sprint iPhone 4S to Tmobile?

I'm planning on buying this phone from a friend but ppl tell me I can't unlock it? Is this true?

You should be able to I think you'd just have to swap out the sim card

Sprint and Verizon iPhone 4S come factory unlocked since they use the CDMA radio, not GSM. You will be limited to 2G (EDGE) data only, no MMS and Siri and FaceTime will only work on Wi-Fi.

Try the website that my friend has used it to unlock his iPhone for permanent.
it seems they are official apple unlockers.their price was about $25 and they did a remote unlock by

getting the iPhone imei number. They unlocked my friend's iPhone in just 12 hours.

take a look at their website: http://officialappleunlocker.com/