As you may know, some countries are blocking access to whatsapp calls, but users usually overcome that with a VPN service, but today even with that powerful internet anonymity solution, ISPs and telecom operators are blocking whatsapp calls even when using VPN on your phone. The question is, how are ISPs able to detect that traffic, as it's encrypted, and ban such voip services. Any technical details would be great, I actually suspect that, it's not just the ISPs whos blocking it, whatsapp it self is doing it too.
Whatsapp uses specific ports including: 5223, 5228, 4244, 5242, 5222. By monitoring packets to these ports, it is very easy for a packet filter to figure out which clients are trying to conduct a Whatsapp session. The problem is made easier because Whatsapp uses TCP, a sequential protocol. Using this protocol makes it even easier to target and interrupt.
If you are using a VPN and the server is outside of the country, then there is no way to block you, other than cutting off the entire connection to the VPN. As of this summer, the government of the United Arab Emirates decided to make VPNs illegal and is now blocking them wholesale. They do this by maintaining lists of known VPN endpoints and then blacklisting them all.
You can evade this restriction by making a custom tunnel out of the country. Be aware that doing so is a felony in the UAE.
It could be due to a DNS Leak
As this useful website states:
Under certain conditions, even when connected to the anonymity network, the operating system will continue to use its default DNS servers instead of the anonymous DNS servers assigned to your computer by the anonymity network. DNS leaks are a major privacy threat since the anonymity network may be providing a false sense of security while private data is leaking.
Knowing that, if the ISP's DNS Servers are blocking Whatsapp calls and you think you're using your VPN provider's but instead you are really using your ISP's then there would be no point on using a VPN because the ban/blocking is is the ISP's DNS servers which you are using. In this case you would have a DNS Leak and the traffic would still be blocked.
You can test if you are suffering a DNS Leak in the same website I linked before and they offer ways to fix this so take a look at that. It may be useful.
This is something happen all the time with me in Saudi firstly I'd like to confirm the local Telecom companies have no authority to access & stop that in your phone . Secondly: make sure you have tried that correctly. Connect the whatsApp by your VPN and retry
sometimes you the issue is from VPN itself, the servers is damage or traffic. try using other VPN it should work.