I have set up Zoiper on my computer. I am able to make a test call to my mobile phone but when I try to call anyone else, I cannot hear them and they cannot hear me.
Which is the OS you use?
WINDOWS and I cannot find a way to not get that message when I callback a number
Try disabling STUN for the affected account .Open the Settings menu, select Preferences and then open the Account section. After that go to "Advanced" section. Now go to the SIP account that you are experiencing the issues with and disable all settings that are not explicitly required by your VoIP provider such as:BLF,Subscribe Presence,Publish presence,Send KPML,Use Rport,Use Rport media,Force RFC-3264Then please use the drop-down menus and set STUN to Do Not use STUN and Subscribe for MWI to Disabled.
Otherwise error 404 is usually returned when the number you are trying to dial is not existing or is wrong.