the subject is what.

Agreeing with faldage, but not finding anything on-line directly on point, I note the following:

Achilles is a lion. --- Who is a lion?
Achilles is the subject of the former sentence. http://www.dictionary.com/doctor/faq/l/linkingverb.html
Why should one think the subject somehow "shifts" in the latter sentence?

What is your phone number?
"your phone number" is the subject complement -- implying that it is not the subject.
http://www.uottawa.ca/academic/arts/writcent/hypergrammar/adjectve.html

The same site confirms that the "subject complement" is not the "subject": "A linking verb connects a subject to a subject complement which identifies or describes the subject"
http://www.uottawa.ca/academic/arts/writcent/hypergrammar/link.html#linking verb