De lo que no cuesta se llena la cesta

[ROBINDESBOIS]

Este dicho se usa bastante cuando alguien se aprovecha de la situación o de algún producto gratis y acapara todo lo que ve, o mismamente llamar por teléfono desde el trabajo porque no lo pagas tú. AGAIN who can provide with an English equivalent?

[hermit]

not really a 'dicho', but it's common in english to say something like:
"she can't resist a bargain".

people who use telephones like that are "freeloaders" or "cheapskates"; these terms can apply to any comparable situation, not just telephone use.

hermit

[Rusty]

Agree with 'freeloader' and 'cheapskate'. Also, 'penny pincher', but this is usually used to describe a person who is stingy with their money or is very frugal.

Here are some sayings that might fit the bill:
If it's free, it's for me.
If it's free, give me three.
People will take anything if it's free.