Bert gets a new telephone.
The funny thing, or the sad thing depending on how you look at it, is that kids today would probably see Ernie at the top of this sketch and say, "Oh, look! It's one of those neat old-fashioned telephones!" Or, more depressingly, they might see Ernie at the top of this sketch and say, "What is that thing?"
I suddenly notice that Ernie's phone isn't plugged in to anything. Haha. Also, not to be nitpicky, but getting a second phone doesn't automatically give you a second phone line. Also, how did Ernie know Bert's new number?
The "what's in the box?" gag reminds me of an old family joke. When I was a kid, we always used to save boxes and then reuse them to wrap presents in, so the pictures on the box could be misleading. It got to the point where it was surprising when the picture on the box actually matched what was inside.