Soemtimes this may be the case. In these instances you may want to drive offline traffic online via more traditional methods. Rather than a phone number, drive them to an opt-in page.
Look into ad's in trade journals, publications, news papers.
Another option is to look into taking out ads in industry related news letters & ezines, contact directly the list owners and possibly pull off a JV with them.
Sometimes there is just not a clear cut solution for some industries, thats when you really need to look at cross promotional type approaches. Explain the benefits and pretty much sell your service to people that might be perceived outside of your niche, but closely enough related to where they would do business with you.
Alot of it depends on your service and what you are trying to achieve. Go where your audience hangs out, are there online forums? Post there... Are there online blogs? Comment there...Take the top 5 professionals in your local area, call them up and ask for advice. No need to make a hard sale, just inquire as to whether your services might be of value to someone in their field...
I know some of this is elementary, but sometimes the most simple solution is the right one....Occums Razor......
Hops I may have sparked some ideas,
Chris