In Twilio, a simultaneous ring is broken in airplane mode.
Since the first call that connects will cancel all other attempts, if there is a line in flight mode, it will immediately pick up and take the user to voicemail.
The fix will be to cancel all other attempts only when the call is successfully connected, and not PICKED-UP. In this example, if 111-111-1111 picks up the phone but does not press 1, call 222-222-2222 should not be canceled:
<?xml version="1.0" encoding="UTF-8"?>
<Response>
<Dial>
<Number url="/screen">
111-111-1111
</Number>
<Number url="/screen">
222-222-2222
</Number>
</Dial>
</Response>
--- /screen
<?xml version="1.0" encoding="UTF-8"?>
<Response>
<Gather action="/complete_call">
<Say>Press any key to accept this call</Say>
</Gather>
<Hangup/>
</Response>
Can this be fixed?
In your document, it seems that you know about this problem. Here are just a Twilio doc:
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https://www.twilio.com/docs/api/twiml/number