I believe they are referring the electronic field created around an answering jack (wrt "field" they could be they are talking about the connection or noise or something)

Literally
"telefoni jack" equivalent to "answering jack" (from ordbogen.com Tek.Engelsk)

Field = felt (Elektricitet, elektrisk udstyr)

English definition of answering jack:
http://encyclopedia2.thefreedictionary.com/answering jack

You can see a few "jacks" in the pictures below:
http://www.pricerunner.dk/sp/jack-kabel-10-m.html

If you are loath to use the English word "jack" you could substitute "jack" for "kabel-ende/tilslutning/forbinelse" (or something like that) - you could be technically wrong, but the general population might be more likely to understand

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Note added at 19 hrs (2011-11-01 05:53:34 GMT)
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telefonbøsning = jack (ordbogen.com)