How to downgrade from Iphone 3gs with xx134 later

I accidentally upgraded an Iphone 3GS which was made later than the 34th week. The baseband is 6.15 and the phone says no service. I would like to know if there is any solution to get the phone to work as a phone. Also if there aren't any solutions present right now, will anything be coming out in the near future as the iphone is as good as a ipod touch now. I appreciate all the answers. Thank you