Look for the “link” icon, and tap it to enable “link” mode. In link mode phone numbers, addresses and web links in your draft will become links. Tap on a phone number to dial it, an address to look it up in maps, or a web link to open it in Safari.
While in link mode, the draft is not editable.
On the iPhone version of Drafts, be aware that the link mode button is only available in full screen reading mode. Swipe down from the middle toolbar to enter this mode.