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:
Use Rport media,
Then use the drop-down menus and set STUN to Do Not use STUN and Subscribe for MWI to Disabled.