Apparently the version of IOWRT probably based on OpenWRT used in the Genexis ED500 router
PURE-ED500-KVIKNET-4.3.6.132-R has a problem with VoIP preventing the use of softphones eg. Zoiper or MizuDROID because a bad algorithm is used.
The problem may be located in OpenWRT, but I am not at all familiar with the source code, so I must rely on somebody with knowledge of the VoIP module.
The case is the following:
The IOWRT VoIP module will accept a call from the SIP server before any analogue phone is picked up and cancel the call when not answered by the analogue phone after some time. This has as consequence, that the subscriber will not be able to answer a call by using a mobile phone app, e.g. when away from the analogue phones.
This is not the behaviour found in e.g. CISCO VoIP products which are working seamlessly with phone apps and analogue phones.
It appears you are using firmware that is not from the official OpenWrt project.
When using forks/offshoots/vendor-specific builds that are "based on OpenWrt", there may be many differences compared to the official versions (hosted by OpenWrt.org). Some of these customizations may fundamentally change the way that OpenWrt works. You might need help from people with specific/specialized knowledge about the firmware you are using, so it is possible that advice you get here may not be useful.
Ask for help from the maintainer(s) or user community of the specific firmware that you are using.
Provide the source code for the firmware so that users on this forum can understand how your firmware works (OpenWrt forum users are volunteers, so somebody might look at the code if they have time and are interested in your issue).
If you believe that this specific issue is common to generic/official OpenWrt and/or the maintainers of your build have indicated as such, please feel free to clarify.
As far as I can tell the Genexis ED500 router is not natively supported by OpenWRT. Nor does OpenWRT have a native VoIP 'module'.
Any questions about IOWRT (or their VoIP module) will need to be directed at the developers of that particular piece of software. IOPSYS / IOWRT · GitLab might be a starting point.
This is not a problem with the router or specially with OpenWrt. The router (either with or without OpenWrt) just transports the data between caller and called device (without any manipulation of this data). To resolve your problems you must look at the VoIP handling software on these devices.