There are two different phone systems: the 33 system and the 2280 system.
33 system: If the operator says "XYZ, you have a call on 33," pick up the phone, dial 33, and start talking. To put the call on hold, press the receiver button quickly so you hear a funny tone (not the dial tone - if you hear that, you hung up), dial 33 or whatever line you're parking the call on (34, 35, 36), and hang up the phone.
To clear an indyme (call button request) on a phone that uses the 33 system, press the small button on the side of the phone.
2280 system: If the operator says "XYZ, you have a call on 2280," pick up the phone, wait for the automated voice to start talking, then dial #2280. You'll hear three beeps, then when you start to hear background noise, you can start talking. To put the call on hold, press the receiver button so you hear the dial tone, dial 2180 or whatever line you're parking the call on (2181, 2182, 2183, etc.), and hang up the phone.
To clear an indyme on a phone that uses the 2280 system, pick up the phone, wait for the automated voice to start talking, and hang it up.