T54W custom ringtone not playing

[VerticalTechnik]

Good day all, I hope this post finds you well.

Maybe somebody can help me with this issue.

We have several T54W with FW96.86.0.77 in our office.
I want to assign our custom ringtone (WAV File) to some of the phones.

What have I done:
a) custom ringtone has been uploaded and configured accordingly
b) custom ringtone shows up on Yealink and been assigned accordingly

Problem:
Custom ringtone is not being played when someone calls

Note:
Within our 3CX System Management
selecting the appropriate device > Telephone provisioning - only Standard Ringtones can be selected, there is no option to select custom ring.

Question:
1. as mentioned above the ringtone is displayed and selected within the Yealink
- should therefore the ringtone being played or does 3CX have any influence?
- or am I missing something?

Side Note:
After trying to change the Custom Ring I noticed that it is not anymore possible to delete or replace the Custom Ring.

Thanks to anyone who can bring some light in to this topic.