Specifying channel mode for speech

Osmo-bts gets the MODE_MODIFY_REQ from OpenBSC with GSM48_CMODE_SPEECH_EFR to use for the channel mode. However, I would like to be able to specify using GSM48_CMODE_SPEECH_V1 instead (which is FR; EFR is not supported in L1). Is there an easy way to set the preferred channel mode/audio support? I didn't see anything on the VTY wiki or in the config file. Thanks, Tom