In order to do this prank you need three things.
1. A phone
2. Someone that is calling you
3. Someone who likes to talk alot and you can't shut them up (perferably someone related to or knowing the person that is calling you.
When you are the phone talking to someone tell them to hold because you need to water your cactus. When they are holding push flash. Dial somebody that is very anoying or someone who won't shut up. Then when they answer. Flash back over the conversation will go something like this:
Friend: Yeah so anyway old man carter started waving his fist in the air as I began asking for ....
You: Oh, I got to water my cactus will you please hold?
Friend: Oh, all right!
You: (Flash and dial the number)
Ring 1 Ring 2
Mildred Monday: Hello?
You: (Hit flash again)
Mildred Monday: Hellooooooooooo? Are you there?
Friend: Hello, George?
Mildred Monday: No this is Mildred Monday.
Friend: What happened to George?
Mildred Monday: George isn't here is this Chris?
Friend: Yeah but...
Mildred Monday: Oh high Chris you were the person I wanted to talk to. Whenever I pee my bladder makes this awful noise....
Friend: (Soundling pissed off) yeah?
You: (Mute the headset so you can freely laugh at poor Chris who will have to stay on the line. If he hangs up Mildred will just call back.) Time them and see how long they will stay on the phone for. My record for this is 15 minutes.