1. To begin talking to someone on the telephone. The full word "telephone" can also be used before "extension" in both usages. The boss just got on the extension, but I can tell her you stopped by.

2. To ask someone else to speak to the caller or join in on the current phone conversation. In this usage, a noun or pronoun is used between "get" and "on." Get Ryan on the extension—he's the only one that can sort out this mess. OK, well, this irate customer is demanding that I get the CEO on the phone extension.