iOS Apps - stopping alert ringing while phone on a conversation

We have a standalone iOS apps that provides reminder alert (with special ring tone) for specific event configured on the apps itself (no external connection at all for this iOS apps). The reminder alert works well sounding out loud alert to individual users on configured date & time. However, when someone is on an important call and the same alert tone sounds loudly, it started to create very frustrated users. Project Requirement: We are looking for a means to configure our iOS apps so that it will automatically TURN-OFF the sound-out when a phone conversation is on-going. This should happen automatically and also automatically revert back to the normal behaviour (ie. alert sound-out when no conversation is on-going). Do-Not-Disturb feature on iPhone is hence, not acceptable for this case. We will consider the requirement achieved with any iOS apps codes given, OR a recommendation of method that we tested OK and working well (we have our own iOS developers). We have checked online for different recommendations but none of them works so far. Some suggestions even said this will be taken care of with iOS Release 14 but that has been proven a wrong information.