Do you have prince albert in a can?
Q: Do you have prince albert in a can?
Q: Well, you'd better let him out!
This is a prank call, similar to the "Is your refrigerator running?" joke. In case you don't know prank calls were a way that kids would amuse themselves by calling people and telling jokes like the aforementioned joke. They only worked well up until the invention of caller ID.
For this joke to be funny, the jokester needs to call a drugstore in the early 20th century, and ask them the setup. The person who answers the phone would answer yes, that they do have "prince albert in a can", since it is a popular brand of chewing tobacco back during that time period. The punchline "Well, you'd better let him out!" indicates that the prankster was not calling about the chewing tobacco, but was instead calling about the actual Prince Albert.
Now, Prince albert was dead by the time that this joke could have been told, since he died before the invention of the telephone. Additionally, the idea that someone would have a person in a can - let alone the prince consort of the queen of England - is pretty absurd.
So, to sum it all up, this is a pun - the words "Prince albert in a can" can mean both a brand of chewing tobacco, and some dead dude, so the drug store attendant would probably have cursed, and then hung up the phone.