I have found that asking in a childish voice "Who is this?" again and again and again until they hang up can be amusing.

Of course, you could ask personal questions of the telemarketers, or just say "wait a minute, be right back" and take the damned thing off the hook for 30 minutes

If the puter is on you can play them sound files in a loop.

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Semibodacious Transmogrifications a Specialty

1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)

"Time flies like and arrow, yet fruit flies like a banana"
---Marx (Groucho)