While there could be multiple reasons for this, the most common reason is that you have called your mobile number with the same number listed as the caller ID. Some mobile phones freeze up if they receive a call from their own number.

For example, if your mobile phone number is +18085551234, and you make an outbound call to that number from your verified caller ID of +18085551234, the call will most likely have a status of "completed", but the phone will never ring.

This sort of issue can also occur if you have a Twilio phone number forwarding calls to your mobile phone using <Dial>. By default, <Dial> sets the caller ID of the incoming call as the caller ID of the outgoing call.

Changing the caller ID of the outbound <Dial> leg of the call to use your Twilio phone number (or another valid Twilio Caller ID) will correct this issue.