Making calls by clicking a phone number in Sell
You can let users make outbound calls by clicking a phone number in a contact or lead record.

How it works
The phone numbers in contact and lead records are activated by installing a Zendesk app that listens to the voice.dialout event in the Zendesk Apps framework. See voice.dialout in the Apps Sell API docs.
When a user clicks a phone number in a lead or contact record, the voice.dialout event fires and returns an event payload. For example:
{"number": "+1234352356","resource_id": 988,"resource_type": "lead","resource_name": "Sarah Smith"}
Use the data (specified as e in the following example) to initiate an outbound call:
client.on('voice.dialout', function(e) {makeCall(e);});
See Working with events in the Apps Developer Guide.