During calls it is recommended, that all VoIP packets (RTP) are sent on the same downstream channel. Theoretically this should reduce jitter. Moreover there have been cases where only a one-way communication was possible with downstream bonding activated.
Using a Cisco CMTS it is possible to schedule all RTP packets for an ongoing call onto one downstream channel only:
This is done by requiring an attribute mask with its MSB (most significant bit) unset and prohibiting an attribute mask with the MSB being set. This bit states if bonding is allowed.