Human ear cannot generally perceive mouth-to-ear delay under 200 milliseconds. 

If you perceive a delay during call, chances are that is between 300 and 500 milliseconds, which is perfectly acceptable to have a conversation on phone.

Higher delay in the order of 1-2 seconds (such as satellite communications) makes phone call more difficult, but still doable with some practice.

There are two main causes for perceivable delay using EVSS:

• network jittering: audio data packets arrives irregularly, with variable delay, depending on network issues and structure. PrivateGSM is constantly measuring jittering and automatically adapts to it, adding a delay buffer, in order to produce a constant and smooth audio flow
• network latency: audio data packets flows from caller PrivateGSM -> PrivateServer -> called PrivateGSM and in the opposite direction. If both parties are far from PrivateServer the transmission of audio data will get a delay.

While GPRS provides enough bandwidth for PrivateGSM to play crystal-clear audio, it exhibits a high jitter, which introduces a perceivable delay.