Why can't I connect the headphones to the app during a call on iOS devices?

When using an iOS device, the app connects to the headphones via the BLE (Bluetooth Low Energy) protocol. During a phone call, the system prioritises the call and allocates most of its processing power and communication bandwidth to it. As a result, BLE communication is temporarily suspended, and the app cannot connect to the headphones at that time. This is normal behavior and not a fault.