I don’t work with cisco very often however recently  had a requirement where I had to change the ptime value coming in from a Cisco from 30 ms to 20 ms

Naturally I searched Google to find ways to change the ptime value in Cisco however could not find any useful links, eventually It turned  out that to change the ptime you have to specify the voice payload size under the dialpeer for the codec


160 bytes for G711 is 20ms

249 bytes for G71 is 30ms