Increase the ring-volume of the rollover line

[phadobas]

Here is another request I need to tackle:
The system is SV8500 and the phone is DT700 series.

The user is on the phone talking to somebody. A second call comes in on a second line (rollover). On this second call, the ring-volume is less loud than it is on the main line. This is fine and it is documented this way in the manual.

But still, I need to make it so the rollover rings the same loudness level as the main line. Per the manual, this can be done, if the user's phone is in Headset-mode, ie, he is talking after he pressed the headset/handset button to switch the phone's audio to the headset port. In this case, the ring volume on the 2nd incoming call is the same as the main line.
However, the user doesn't want to wear a headset.

Any ideas how this can be solved? There is no settings for this - that I could find - in the phone's menus, much less anything in the manual...

 

[belevedere]

Does the handset work plugged into the headset jack?

 

[phadobas]

Hmm, it does work. Interesting. That might just be the solution - as long as the user is willing to not only pick up his handset but also to press the headset button...