We have been asked for something like this as well, just not phone numbers.
We got legal advice which, in Australia, was, as long as the website is publically available AND you don't upload any code to their server in order to achieve it (like you just request the HTML) AND you don't impact their performance (like you throttle it) THEN you as the developer are merely accessing the website using a technology like a human in a browser OR a search engine would ... (BUT I'M NOT A LAWYER, GET PROFESSIONAL ADVICE)
We then went through and checked the terms of service for the target sites which didn't expressly forbid it as part of the terms of using the website.
Once we had done all that, we still weren't that comfortable with doing it, so we priced ourselves out of the job.
Ethics wise ... well ... there is not usually a clear straight answer ...
The people whos details you are collecting haven't given you (or your employers) permission to republish / use the data ... So points off here.
But they have given their details to this site so in theroy they could hook the predictive dialler to the website in real time and simply act like a human consuming the service ... Is this any different? It feels better to me for some completely unexplainable reason ... given the net result isn't any differnt I would give points here as its not changing anything
Enabling telemarketers to annoy me at dinner time ... major points off here.
Feeling the need to ask the question ... puts it in the grey/points off catagory.
All in all data harvesting isn't a real issue, google would be in trouble if it was. Each site may have rules around usage (like stackexchange for example) but you have to make a call on the ethics.