Hi,
I understand your requirements and I believe you want to have an arduino gadget with two buttons for 'call pick-up' and 'call-end' function.
And you want only the arduino code and not either of the mobile app code.
But in practical application, the arduino can only send two strings: 'pick-up' and 'terminate'... and it's up to the mobile app to interpret these strings.
Both Android and IOs has security features that prohibits KEYWORDS from being auto-executed in their OS once that keyword is received either by BLE, wireless, NFC or RFID.
I don't do IOs but I can show you how we do it in Android.