VOIP no voice answering after 4 rings/20 seconds

seems to be not you only:
https://forums.grandstream.com/t/ht802-disconnecting-outbound-calls-after-20-seconds/31130/14

try using a sip client instead of the grandstream ata or an ip phone directly and see if the issue persist

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Do you have the kmod-nf-nathelper-extra package installed?

If you do, have you enabled it in sysctl.conf?

net.netfilter.nf_conntrack_helper=1

Analyze your traffic first, then "tweak router configuration".
There is no single technical evidence yet that your router is doing something wrong.
OpenWrt is an IP router, but your problem description "there is no sound on either end" has nothing to do with IP. Once you will see that some TCP or UDP packets are lost, misrouted, etc - then it will be appropriate to discuss this here.

I have kmod-nf-nathelper-extra, without it, SIP/RTP would not work at all.
By default this module adds the following file in /etc/sysctl.d:

11-nf-conntrack.conf 
# Do not edit, changes to this file will be lost on upgrades
# /etc/sysctl.conf can be used to customize sysctl settings

net.netfilter.nf_conntrack_acct=1
net.netfilter.nf_conntrack_checksum=0
net.netfilter.nf_conntrack_max=16384
net.netfilter.nf_conntrack_tcp_timeout_established=7440
net.netfilter.nf_conntrack_udp_timeout=60
net.netfilter.nf_conntrack_udp_timeout_stream=180

the parameter you suggested isn't there, but the module must be enabled because otherwise SIP/RTP would not work.

AFAIK, and from what I have read, the conntrack helper is not enabled by default.

Conntrack helper should work by default in the LAN-to-WAN direction:

iptables-save -t raw

It requires to restart the firewall service after installing the package:
https://openwrt.org/packages/pkgdata/kmod-nf-nathelper-extra
But it may not be enough in some cases which can be covered by sysctl.

I tried too with a Grandstream DP715 phone, same result

I do not know how many times I restarted since I included nathelper a year ago. Beside I prefer to build my own firmware selecting packages in the firmware configuration

It must be, because if I build the firmware without it, VOIP does not work behind NAT

Not that hard to just try and find out if you are correct

Hello, I use a Cisco SPA 112 it is very similar, if you connect your device by ethernet you should not have a problem of any kind, verify in connections that the IP of the device is connected to an udp port in my case 5060, if so, no It is a problem of the router, it is a problem of the provisioning of the device {configuration}. ask your ISP for access credentials and be sure to modify the phone's tone numbers before stopping. With a stateful firewall, this is not the problem

Some ISP's never reveal VoIP setup details to customers so we are forced to get them sniffing VoIP traffic.

I had ling time VoIP NAT issues similar to the OP but after installing the stated kernel netfilter modules it seems to be solved, touch wood.

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Hello, I hope you are lucky and everything works fine. Is the problem strange, does it work with its own router behind the ISP's router?

With the ISP’s router it works perfect.

Now you are with your router, behind the ISP router?
In what environment do you work?
(ISP Router) ----- (Your Router) ----- (Voip)

ONT —- Openwrt router + VoIP adapter

The ISP’s router also when used connects to the ONT.

I understand, you have your router by pppoe. It does not have double NAT problems, with a standard openwrt image you should not have problems, these devices are to connect and work, it only synchronizes and can be used, if you have checked the connections made by your router and the IP of the device to established a connection to a udp port, that's fine there, check your firewall.
I still think that it is a problem with the configuration of the device.
Reinstall the ISP router to have technical support and explain the problem to reconfigure the device

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At the moment I don't have any problems with my VoIP traffic.

My connection is fiber with CG-NAT so more than likely it's double NAT.

VOIP works fine with the ISP (Vodafone Italy) router. Also the ISP provides all VOIP parameters necessary to configure my own voip box (they are mandated by law). In my case, re-installing the ISP router, is not an option as it does not support most of the services I need, like openvpn, fail over to 4G, etc.

Thank you all for your suggestions, however I had already tried everything that has been suggested so far before posting (I have had this issue unresolved for nearly 2 years now, so it is hard to imagine what I have not tried in that time). I have tried every possible tweak of the HT802 parameters as well as numerous OpenWRT configurations.

The issue with the call being silent (no audio) if I reply after 20" (4 rings), persists. I have tried also with a different IP Phone (still a Grandstream), same result. What I have not tried is a different make/model voip box.

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