The slide does not have wifi calling yet, but will if the 2.2 update, that exists in at least two versions, ever gets sent out by tmo. If you root and flash a custom rom you can use wifi calling if the rom supports it (e.g. Cyanogenmod or one of the leaked 2.2 espresso/mytouch slide update roms).
Find a post useful? Show some love and click the "Thanks" button.