I know all throughout the show they have to climb the telephone poll to answer and use the phone, but I was watching an early episode the other day and they were actually using the phone in the kitchen.... did they ever explained why they stopped using that one in favor of climing the poll?

Arnold chewed the connection cord??

In the first season they put the kitchen phone on the wall but never ran the wires because they only had enough to reach the pole.

In a few early season 3 episodes they actually used the phone in the kitchen due to the Monroe Bros. cutting a hole in the kitchen ceiling letting the wires reach. After a few episodes of Oliver complaining about the hole he had them patch it up and they went back to using the pole.

Strangely I noticed in the sixth season the phone wasn't there anymore. They never explained what happened to it.