- Check both of you have turned that call type on in Calls.
- Allow camera and microphone access — your phone's Settings → CalmHandoff, or the padlock in your browser's address bar.
- Make sure the other parent's app is open, or that they allow notifications.
- Try mobile data instead of Wi-Fi. Office, school and hotel networks are the usual culprits.
Why the network matters
Calls go directly between the two devices, which is why there are no per-minute fees and no server in the middle holding your conversation. Roughly one network in eight blocks that; those calls fall back to a relay automatically. A few very restrictive networks block both.
If it still fails on mobile data, email support with the rough time you tried — the attempt is in the call log and we can see where it stopped.