as in "den Spieß umdrehen" - might work in this context

Not sure that helps answer your Q though: the function of „auf“ in “auflaufen”?

They are letting the companies think they are interested so that the company then sends them information, which is then used as evidence against the company.

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Note added at 1 hr (2009-10-29 10:52:02 GMT)
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Example: "Gerade die Details müssen hierbei beachtet werden; die bieten die besten Möglichkeiten, einzuhaken. Da brauchen auch gar keine besonderen Tricks angewendet werden. Man muß die Leute einfach auflaufen lassen, indem man ihnen gegenseitig Widersprüche in Kleinigkeiten vorhält ... und dann weiter in sie eindringen, bis schließlich ihr Widerstand zusammenbricht." (See http://bidok.uibk.ac.at/library/brusten-vernehmungsmethoden.... .)

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Note added at 1 hr (2009-10-29 10:56:48 GMT)
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Here is some more support for my suggestion:

"My Pons Idiomatik Deutsch-Englisch dictionary offers the following:
to let s.o (get on with it and) run into trouble

My suggestions are:
to stand by and let someone screw up big time/royally (informal)
to let someone mess up on their own" (See http://dict.leo.org/forum/viewUnsolvedquery.php?idThread=169... .)

Just another idea - which might work for a title.

Collins German Dictionary
jdn auflaufen lassen = to drop sb in it.

i.e. let them think they have got your interest, then drop them in it (hot water, etc.). They think they have achieved their ends and then find themselevs in legal trouble.

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Note added at 18 mins (2009-10-29 10:07:26 GMT)
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Similar approach found in English:

South Ayrshire Council News
Drop In And Drop Them In It!
Have you ever received a letter telling you that you have won a free prize but asks you to pay something first? Ever received an email from someone that appears to be from your bank or building society asking you to provide personal and financial details even though they should already know them without asking? If so, you have probably been the target of scammers. Scams are becoming an unwanted fact of life and are estimated to cost the UK £3.5 billion a year.

South Ayrshire Trading Standards have joined forces with the Office of Fair Trading to launch a 'Scamnesty' campaign asking the public to help in the fight against scams.

As part of Scams Awareness Month, the Office of Fair Trading in partnership with 81 local authority Trading Standards Services is asking members of the public to collect any potential scam mailings they have recently received and drop them into designated 'Scamnesty' boxes at local libraries and other public areas across the country. In South Ayrshire boxes will be posted in the County Buildings and Burns House reception areas, the Trading Standards Advice and Intervention Centre and in all libraries throughout South Ayrshire. The boxes will be recognisable by the message "DROP IN AND DROP THEM IN IT".

http://www.south-ayrshire.gov.uk/news/Drop-In-And-Drop-Them-...


Another explanation of the idiom:

The rules of work: a definitive code for personal success - Google Buchsuche-Ergebnisseitevon Richard Templar - 2003 - Self-Help - 219 Seiten
... but it also means you don't spy on your colleagues, rat on them, betray them, stitch them up, shop them, grass them up or drop them in it. ...

http://books.google.de/books?id=sV6hawqRjBkC&pg=PA103&lpg=PA...

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Note added at 56 mins (2009-10-29 10:45:37 GMT)
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The idea here is close to that of "lulling them into a false sense of security".

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Note added at 1 hr (2009-10-29 10:54:38 GMT)
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"Hang them out to dry" could also work

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Note added at 23 hrs (2009-10-30 09:06:15 GMT)
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If I was starting from scratch, I would give the following answer:

"Give cold callers enough rope to hang themselves!"

As this is the title, a play on words seems apt.

the cold shoulder = a show of intentional unfriendliness" (Oxford) just what one should show when these people disturb you
also as verb : to cold-shoulder someone

or, similar but emphasizing the "fall" part:
let someone's actions backfire / blow up in one's own face

I think "to set someone up for the fall" might be one of the closest English almost equivalents.

You asked what the part "auf" does in auflaufen.
In this particular instance I would also see it as letting them come close (letting them send their information) until they run into/against a/the barrier/(a) ground (kind of like a body check) -= the caller goes to court.
It's almost like entrapping /leading them on /leading them on the ice and then let them hit their own wall/ or, actively: cook their goose/sock it to them/punch, hit them hard- (but it's the bad guys that are being entrapped here, so maybe it's not the right word) A positive term for entrapment maybe?
setting them up for the fall

Einfahren lassen are closely related term -
to really get somebody good / let somebody have it

The challenge seems to find an English equivalent that conjures up the same idea as leading somebody on and letting them run into a barrier.

let somebody run afoul (of) is a similar term but it's a bit different - see below

http://www.answers.com/topic/run-afoul-of



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Note added at 1 day16 hrs (2009-10-31 02:43:13 GMT)
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corr. leading them on the ice where they then slip and fall

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Note added at 1 day16 hrs (2009-10-31 02:45:50 GMT)
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"do somebody in" came also to mind.