"Hold" + going back by "Resume" - audio is muted

[Babylonia]

New update in this matter. And knowing the cause of the problem + solution.
It could be of help also for other users, if they met comparable issues.

As a system admin for a none profit organisation, I am testing several network set-ups “privately” within my own home network, to see if adding new devices could be implemented for the more big network of the organisation itself.

As we are planning to move in the next months to another building and shall be a combination of some organisations together, I am preparing that there shall be a need for several fixed (VOIP) telephone lines.

As for testing:
In my home situation mostly I am using a “NAT behind NAT” router set-up.
One router from my ISP, and a more dedicated router for my own network.

ISP routers often do have limitations and “sealed” options that not can be changed.
One of those “limitations”, is that they offer a VOIP telephone line within the modem/router itself
(+ VOIP ATA conversion within the same modem/router to connect by analogue phone devices).

Even if you don’t make use of the VOIP possibility of the ISP itself, the used 5060 port is locked, and can not be freed up for behind alternative VOIP services.

In spite of the situation, the alternative "Dutch" VOIP provider that I am using, also is compatible and using some alternative VOIP ports (e.g. port 5070), and by this I have set this 5070 port within the Yealing W60P base station, and T41S. There is a limitation that typical the system by this VOIP provider doesn’t seem to work well by the “hold” function, and "resume" afterwards. Strange enough two other Betamax “call out” VOIP services, have no problems using port 6000 as for “hold” and “resume”.

Just by taking out the own ISP router with the “closed” port 5060, and connect my own router directly to the fibreglass internet connection port (by changing some VLAN settings), and setting up all ports within the Yealink devices to port 5060, all is running flawlessly without any problem or glitches.

I can hold and resume, also delegate calls to other VOIP-phone or handset.

So "Mission accomplished".