It's in the programming menu. Be very careful with things that are changed there as you can corrupt the programming of your phone and (possibly) brick it...
Apparently the DI has a choice between EVRC and EVRC-B
Vocoder Discussion - Android Central Forums
Procedure:
Go to Phone and dial ##PROGRAM and press Call.
The password is six 0's (000000)
Go to NAM CDMA Settings
Change "Preferred Voice service option for Home Page", "Preferred Voice Service option for Home Origination", and "Preferred Voice Service Option for Roam Origination".
Then press Menu and hit the "Commit Modifications" option.
REMEMBER: CHANGING RANDOM SETTINGS IN THE PROGRAMMING MAY CAUSE YOUR PHONE TO STOP FUNCTIONING. Also, EVRC-B is not available in all locations, so you may need to switch back if your tower doesn't support it...
From the earlier thread I linked to:
Enhanced Variable Rate CODEC (EVRC) is a speech codec used in CDMA networks. It was developed in 1995 to replace the QCELP vocoder which used more bandwidth on the carrier's network, thus EVRC's primary goal was to offer the mobile carriers more capacity on their networks while not increasing the amount of bandwidth or wireless spectrum needed.[1]. EVRC uses RCELP technology, which Qualcomm claims improves speech quality with lower bit rates
Enhanced Variable Rate Codec B (EVRC-B) is a speech codec used by CDMA networks. EVRC-B is an enhancement to EVRC and compresses each 20 milliseconds of 8000 Hz, 16-bit sampled speech input into output frames of one of the four different sizes: Rate 1 - 171 bits, Rate 1/2 - 80 bits, Rate 1/4 - 40 bits, Rate 1/8 - 16 bits.