This has happened to me. When I flashed to the Stock Sprint Rom. What you need to do is update to a metro PRL file.
OK here it goes.
1, go to this site and get metropcs prl file.
http://www.corolada.com/prl/metropcs/
#### Quick note I'm using the 02001.prl ####
2. Save prl file you get to the root of the SD card.
3. Update the prl file on the phone.
**This codes may be different on your phone**
Go to the dialer and press
##778#
Pick edit from the next window.
Enter the unlock code then enter.
000000
In the next screen click on view info in the lower left corner.
Click on PRL.
Click on Update PRL File then click on the menu button on the lower right side.
Choose edit.
In the next screen hit browse.
It should show up the PRL file you downloaded in step 1
pick the prl and hit ok when you get back to the older screen.
in the next screen click on the menu button in the lower right side of the screen and hit OK.
Hit yes when it ask if you want to update the item
hit OK to reset the phone.
When it comes back the phone should be able to dial out and work.