i've been in a lot of pages, guys, you are my last option, this is what's happening (i read the sticky, but was not helpful)

a friend of mine recently bough a 3g with the 4.1.2 software, the thing was not working, so i downgraded to 3.1.2 but the baseband remained 6.15.00, i need this 3G to work with any carrier here in the caribean.

I have tried using redsn0w 0.9.14b2 to downgrade baseband but i keep getting an error (IPSW not supported) so im a little lost here

phone specs:

Iphone 3g
ipsw 3.1.2
baseband 6.15.00 (need to downgrade this)
currently not able to make calls.

Any help will be highly appreciated, tanks

If having iOS4 is not essential for you, I recommend you install Whited00r unlocked version from here iPhone 3G
It's a custom 3.1.3 firmware with cool iOS5 features, and it runs really smooth.
I used it with 06.15.00 and everything was working fine.
Also, it will be pre-unlocked, so it will be running with any carrier.
You can restore it directly through iTunes.
Hope this helps.