Frame relay sub-interfaces do not have simple Frame Relay map classes applied to reflect their individual bandwidths. If you do not create a class map and apply it to each of their DLCIs the router will assume a CIR of 56K.
This issue will cause output drops on the main serial interface and will cause choppy voice even if a service-policy for Low latency queuing (LLQ) is applied to the interface
Create a frame-relay map-class and apply the LLQ service policy and the appropriate frame relay CIR.
For further information, refer to: VoIP over Frame Relay with Quality of Service (Fragmentation, Traffic Shaping, LLQ/ IP RTP Priority)