2nd day fighting with issue, not able to find it's source. 

We developed 2 SIP apps - for Android and IOS (using PJSIP)

For Android all fine :) 

For IOS - when remote party hangs up - FS does not send BYE message. 

There must be something I'm missing, when doing either INVIE or else - but I can't find any serious difference between Android and IOS invites.

Maybe someone have a clue, how it's possible to ask FS to not send BYE?