Unlocking just for voice on Tmobile?

[KenDude]

I have a 1 year old iphone 3GS that I've upgraded along the way to the current 4.01. It seems that if I want to use it on Tmobile that I have to somehow downgrade the phone first. I'm not sure if my prior upgrades are reversible or not, it always said it was backing up the phone, but I don't know where these image(s) might be located if they do exist, any advice?

Now I should be clear, I don't want the iphone on Tmobile with voice, data, text, etc. ALL I NEED IS VOICE. Yes I plan to give the phone to my 6 year old son and he just wants it to play games on and to call me with, therefore, I thought I might get a new Samsung Galaxy S phone as my primary phone and then get another SIM card for tmobile and put it into the iphone if it would allow him to make phone calls and put him on a shared plan with me.

Is this possible? What's the best way to go about doing this?

[ianbroste]

First off thats a lucky 6 year old.

If the phone has never been jailbroken before and you saved your shsh blobs (clicked "make my life easier") or you never manually saved your shsh blobs for 3.1.2 or 3.1.3 then you cannot currently jailbreak/unlock the phone. A jailbreak and unlock should be coming out which will hopefully solve this problem (you aren't alone). When there is a jailbreak/unlock you will just need a pre-paid T-mobile sim card to have voice only. Your son will still be able to use wi-fi on the phone but no data.

[6309phone]

Can the 3GS be downgraded from 4.01 to 3.1.3? If so, then Ken can use Spirit and ultrasn0w to unlock and use his phone on TMobile.