On T-Mobile, the Z10 has a fantastic app that let's you place calls over a WiFi. You might ask why would I need it, since their program is unlimited talk/text ? Often I'm deep inside a concrete building (usually a customer's site) where the signals from the cellular towers on the streets cannot penetrate. I whip out my Z10, and make calls (using customer's guest WiFi), and they are left with their mouths open. I was also at a hotel in a remote location and this feature saved me.