It's under Article 60 Section 2 (A)
Like I said, I may be wrong.
When I was forced in, I was always paid premium rate.
When I had enough seniority, I never signed the list because I would take the day off instead.
For instance, our supplement says we get premium rate if we are required to work on Sunday. During peak, they put up a list if anyone's interested in Sunday sort. Everyone who signs that list is paid at straight time because they voluntarily asked to work. Those 8 hours are valuable to FT drivers because they count towards your 60 DOT hours. The word "required" in the language is very specific.
(The one thing that is screwing me up is the Sunday before Xmas that we worked. Everyone that came in and worked volunteered, yet they were all paid premium rate. I don't think this was a contract thing but more of an incentive for more drivers to come in.)