T43-U incoming calls do not land on the correct line/button

[rtfm2022]

Dear All,
i've been trying to test and use Yealink phones for our organization and i am facing the following problem:

I have set up a Yealink T-43U phone with 8 lines.

Whenever i have incoming calls on lines 1, 2, 3, 4, 7, 8 then the correct and corresponding line/button flashes.

However, whenever i have incoming calls on lines 5 or 6 then the call lands on Line 1. If i disable Line 1, then the calls land on Line 2, etc. Only when i disable line 1-2-3-4 the correct Line/button flashes.

I have tried to change the sequence, but the same happens! I guess it has to do deal with the specific numbers, as they are from the same ITSP, but what could be thee cause? If there is something wrong with those 2 numbers, why do they accept calls normally when i disable all other lines?

Can anybody help us?