Hotspot and SIP/VoIP

I am trying to make VoIP/SIP call from a RouterOS hotspot. It works with some SIP providers but not with others so it’s probably a problem between the various NAT/firewalls involved.

The softphone register correctly with the BPX but when I try to make a call, there is no sound (both directions).

If I disable the hotspot on RouterOS but keep all other settings, including the SNAT, everything work fine.
If I accept the address of the BPX in the “walled garden IP list” of the hotspot, then it works if I am not authentificated on the hotspot. If I do the authentification, then it stop working.

Is there a way to use firewall rules to bypass the hotspot when trying to reach the address of the BPX? A way to get the same result, when authentificated, as when using “walled garden IP list”? (I know it’s not a very nice solution but I would be fine with a way of just bypass any hotspot firewall rules/nat when trying to reach a specific destination)

Based on more tests, the problem is that there is no “SIP helper” for hotspot authentificated users.

Is there any way to use the SIP helper with hotspot authentificated users?
In “hotspot → service ports”, there is only a FTP helper…

Is it possible to use firewall rules to use the “firewall → Service Ports” helpers for hotspot authentificated? Even if that mean bypassing totaly the normal hotspot rules to reach a given destination?