Our online calculators (http://www.voip-calculator.com/calculator/) have a packet duration field which refers to the length of voice samples that are buffered up until they are transmitted in an IP datagram. The overhead in VoIP is the bandwidth required by the TCP/IP/RTP headers, and these are sent for every datagram. Obviously, the longer the voice samples are buffered before sending, the fewer header will be transmitted per second, and the more bandwidth efficient the system is. The trade off is that by holding back voice samples, the end to end delay of the system increases, and voice is very sensitive to delay.
A typical figure is 20ms, which results in 50 datagrams being sent per second. The second part of the parameter shows the number of actual voice samples which that duration includes. This is a function of the coding algorithm used. For example, G.729A samples voice once every 10ms. So a 20ms packet duration means that each datagram hold two voice samples.
To find out which packet duration or frequency you use, you will need to check your system configuration, or ask the manufacturer or maintainer.