"[year] called" is a common way to mock outdated objects or behaviours. It's usally followed by "...they want their [mocked thing] back". In this case, it's about the clothes of the guy.

The punchline is, instead of mocking him, he's telling him about a major event of 2001 - the 9/11 terrorist attack. It could imply an actual call through time, but that's not important. The comic basically just wants to subvert your expectations and drops something dark instead of the usual punchline.