Just fyi, POTS is digital up until the very last mile. While the network is digital, it uses time multiplexed serial PCM streams under the hood. Aside from the delay imposed by fiber transmission at the trunk level (~40 milliseconds from one side of the US to the next. If you're using a long distance network that packetizes the connection or sends a call on some weird route, that can certainly increase), the g.711 encoding delay should be fixed at 0.125 milliseconds.