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As long as you have wi-fi, yes. Before you cancel service, you might want to port your number to Google Voice so you don't have to change numbers.If I have an unlocked Samsung Galaxy S 4G and no plan with Tmobile anymore I can still use the phone with a SIP provider like Skype, Vonage, (maybe even Google voice?) etc.?

I've got an SGS 4G and I'm using it just fine with Google Voice (GrooVe IP for calls, official Google Voice app for texting). No issues whatsoever. Have a SIM on there (which I only used for 1 month to test for coverage) but it's been deactivated since December.The SGS 4G has native wifi, but it goes through TMO. It works, but not quite what you want. You would have to make sure there was no SIM in the phone.
You don't need to root at all. You just need to download some apps from Google Play. As for tethering, you can create a wireless hotspot (as a sort of instant WLAN) but with no cell service, what exactly are you going to tether it to?What if you bought a phone, never intending to get a provider? Could you root it and what not, to get calling, wifi, tethering, etc.?
The protocol used by GrooVeIP is the protocol for Google Talk (works with voice, too). Basically, you just have to check the box next to Google Talk/Chat on your Google Voice account settings page in order to be able to make/receive calls on GrooVeIP. That's it. On Android, you can have GrooVeIP take over the native dialer so any call you make will go through GrooVeIP/Google Voice.I read up on GrooveIP. Says you have to go through chat. How does that work?
Do you actually do a voice to voice conversation?
Assuming you're using GrooVe IP, it's live calls using the GV number. That's the reason you need to enable Google Talk in the GV settings. No idea how the T-Mo wi-fi calling works as I've never used it.Is it a live call or a callback type call? Will it automatically use a GV number? I looked at the GV settings on the phone, but primary # was still TMO even though the GV # was listed farther down on the page on the phone.
GrooVe IP doesn't do texts but the official Google Voice app does. That's what I use. GrooVe IP for wi-fi calling and official GV app for texting.Does it do texts like the carrier line does?