What's the issue?

Using amp-bind, trying to populate the href attribute of an tag with a FaceTime link produces an error:

%s: Expression evaluation error in \"%s\". %s amp-bind 'facetime:+' + userData.phone \"facetime:+1-463-123-4447\" is not a valid result for [href]. 

Note that constructing a URL in the same fashion with a sms: protocol works fine.

FaceTime links are documented here: https://developer.apple.com/library/archive/featuredarticles/iPhoneURLScheme_Reference/FacetimeLinks/FacetimeLinks.html#//apple_ref/doc/uid/TP40007899-CH2-SW1

How do we reproduce the issue?

See this jsbin to repro: https://jsbin.com/bokesot/edit?html,console,output

What browsers are affected?

All browsers and devices

Which AMP version is affected?

It was always broken AFAIK