Grandma, open the email I sent you and click the link [or] Grandma, Go to "tinyurl.com/5yp7q".
versus,
Grandma, download Skype, find it, run it, walk through the Installer, make an account, login, add my username, then call me. (and even then, this is glossing over a gross number of details that are hard for the unsavvy).
Even if it's already set up, there's no barrier to use of a simple WebRTC-based service assuming it's done correctly.