Need professional VOIP OTT App -- 2

Hi Friends, I need professional mobile had experienced working with OTT APP (have features call look like messenger/Viber/Whatapp) , can make call and wakeup call anytime when receive push notification of server. We use SIP Server ( Freeswitch) to develop system. Each time have call from user A -> user B, system will push notification to user B User B install mobile app ( Android/IOS) and wakeup and receive call. We can make this flow work with some device but not all device work smooth, some device can not wakeup ( android, ios too) i think the app like that ( voip ) need some special skill and tech to implement. My requirement: please see the attact file