Ok, sorry or the wait. This link will show you all that you need to know about this topic. Your Xtransceiver will contain Yancy's number (the option to call her) randomly.
My recommendation is travel to and from Route 16 to Route 6 on foot. Check your Xtransceiver when you get to all the places, excluding the Dirftvail Drawbridge. all you need to do is to enter any of the places listed in the link and you get a fairly low percentage of Yancy appearing as a contact on your Xtransceiver. I saw somewhere that is was a 5-30% chance.
I hope that I helped.