Do sites like Yellowpages have an obligation to help enforce my trademark?

[Zigner]

When we send out Cease and Desist it includes to discontinue use of all advertisements that infringe on our trademark, business listing sites like Yellowpages included. I am not accusing Yellowpages itself of infringement, I am however asking that, as a third party involved, do they have any responsibility to assist in a situation like this?

What does your corporate counsel say? This is really a question for that person/entity.