If none of the above steps solve your issue, you will need an expert network assessment to determine how much latency, jitter or other issues are actually affecting your phones.
Latency is the amount of time list of egypt whatsapp phone numbers packets are taking to traverse between callers, and jitter is the result of variability in latency between different packets, which can cause packets to arrive in the wrong order. If details like these are outside tolerable limits there are still some other solutions you can apply in-house, but they require more specialised networking skills. They include bandwidth reservation, policy-based network management, and multi protocol label switching.
There are also some specialised items of VoIP hardware that could be installed to rectify specific problems, such as a jitter buffer.
In just a few cases your VoIP provider will be at fault, perhaps through having purchased poor quality termination routes across the internet. You can avoid that problem by choosing a reputable tried-and-tested provider such as IDT at the outset.