There’s a certain feeling I get in the pit of my stomach when I’m waiting for an important call to come through. You know the type – maybe a call from your boss, a potential new employer or news of a loved one who’s due to give birth.

In these situations, I usually stare at my phone, willing it to ring. I make sure – over and again – it’s not on silent or “do not disturb” mode. When the screen is out of my sight, I imagine I can hear the familiar ringtone.

Then it pops up – the missed call notification. But the phone never rang. What happened?

When making a mobile call using 4G or 5G networks, the caller dials a number and their network operator (Telstra or OneNZ, for example) routes the request to the recipient’s device.

For this to work, both phones must be registered with an IP Multimedia Subsystem – or IMS – which automatically happens when you turn on your phone. IMS is the system that allows........

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