My ISP is Vox and they provided a Mikrotik hAP ac lite routerboard
I am able to make calls from the handset but when I try and phone in there are problems.
I see you have mentioned that you hav a Mikrotik there. Could you possibly check if the following settings are disabled as its seems like it migh be becasue of some settings that should be disabled on the outer.:
SIP ALG/ ( On MikroTik routers its called SIP Direct media )
These setting should be under IP - Firewall - service ports and all of them should be disabled.
Please let me know :)