Hello guys,
Thank you once again for your time and effort.
Just to note, I marked @rhbirkelund answer as solution, as I already tried everything as @rhbirkelund, but was still having issues. Seems that the problem was faulty switch, that is all. No matter how you set up:
On the uplink port of the Switch to the MX you set the Native vlan to 1, and allow vlan 90 (and others) if so.
On the port connecting to the VoIP phone you either;
* set it to switchport type trunk with vlan 1 as native and vlan 90 as tagged
* set it to switchport type access on vlan 90
* set it to switchport type access on your data vlan and voice vlan to 90.
It should work and works on all switches except on the one that by coincidence I was using for testing.
Thank you very much, hope this helps someone else in the future. You guys are awesome💪.