You can use FRF.12 or FRF.11 Annex C on PVC basis only.
I do not think you can interleave voice and data packets between different PVCs.
Your large data packets will be fragmented within software queue and interleaved with voice packets at exit of software queue. Once packets leave software queue they will enter hardware/transmit queues which are in your case 2 FIFO queues for FR interface. There are ways if prioratising one of the dual FIFO queues.
Other option is to use FR PVC Interface Priority Queuing [PIPQ] which will prioritise Voice PVC over Data PVC. But unfortunately in this case you have to use strict priority queuing mechanism only, which can harm your data flows.
Your FR PIPQ will take precedence over FRF.12.
If you have low speed links why do you want to use different PVCs for voice and data anyway? Use single PVC and smart queuing mechanisms instead.
Regards,
Vakhtang