We've been thinking along similar lines with Sending a text message (SMS) or email from a form in Collect! Unfortunately the addition I made there has a bug that means this isn't supported. I've issued a fix at https://github.com/opendatakit/collect/pull/2386
After this change, any implicit intent (such as ACTION_DIAL) would be supported as long as a URI of the correct scheme is provided as the uri_data parameter value. For what you've described, that would look like ex:android.intent.action.DIAL(uri_data='tel:555-5555')
Here's a full form that shows a scenario like the one you've described: https://docs.google.com/spreadsheets/d/1AyHr87z7iGzmpYw8Yokdfo7jL2WSm8ScR44gSJOxtFU/edit#gid=0