To publicize a series of teleconferences, you have to get creative. Notice we said publicize, not advertise. Of course you can take out ads in trade publications and newspapers, but if your budget is limited, you may want to save those funds for another venture.
Instead of taking out ads in publications, you can write letters to the editor when you see an article about a topic your teleconference will address. Of course you have to mention the teleconferences in the letter! But don't make it like a commercial. Make sure you concentrate on relevant commentary about the topic at hand and mention the teleconferences as just one way your organization keeps the public informed.
Make sure that you talk about upcoming teleconferences every chance you get. When you meet people at conferences, happy hours and networking events, don't forget to tell them about what your organization is doing. This seems so simple, but you'd be surprised at just how often we all forget that word of mouth is the best way to advertise. Even if you meet someone and think that they are not in your industry, tell them about the teleconference anyway. They may know someone else who'd be interested.