1) 3G JetStream
2) S-Off (get yours from HTCDev now!)
3) Root (get yours from DoomLord)
Using root explorer or your favorite text editor edit the build.prop file under /system/build.prop
You could also use Prop edit (free in the market).
edit the following lines:
ro.ril.reject.cs.ss.enabled=0 ro.ril.reject.mo.ussd.enabled=0 ro.phone.function=1
ro.bt.profiles = 4270403 service.brcm.bt.ag_supported = 1
Look under the programs menu you will see the phone app .
You will now be able to make and receive calls.
Tested on AT&T HTC Jetstream With Dutch "Hi" simcard!
If this helped you Just Thank me !
Reports are rolling in that calling works but sound doesn't work, not through speaker/Bluetooth or normal.
As I have only found these buildprop tweaks, I don't really know what's going on behind it!