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

[Babylonia]

Having the same issue by T41S desktop phone
as by W60P + W56H handsets.

When using the "Hold" key, the caller do get "music" (fine).
Pressing "Resume", connection is still active but all audio forth and back to caller is muted. Display is showing "talking", so in the right mode.

As I have the same problems for two total different devices (T41S and W60P + handsets), I think I have forgotten some basic settings to enable within the webbrowser based menu's. However don't know which setting, when reading the manuals.

Same problem when trying to redirect a call to another handset.
After redirect, audio is muted.

Please do give some advise, or even better, a solution.

W60P
Firmware Version 77.83.0.25
Hardware Version 77.0.0.48.0.0.0

T41S
Firmware Version 66.84.0.15
Hardware Version 68.0.0.0.0.0.0

W56H
Firmware Version 61.83.0.10
Hardware Version 61.0.0.11