Many of my clients are keen to save money by using automated telephone conferencing in conjunction with their web seminars. Large enterprises often have a company-standard phone conferencing system in place with pre-assigned phone numbers and access codes for each employee. Using an automated system is probably not a good idea if you are planning to take questions over the telephone. An operator-assisted call lets the operator get caller information, check their volume, queue them up, and announce each questioner in turn.
But if you are taking your questions via a type-in panel on the web conferencing vendor's technology, the use of a live telephone operator is probably overkill. You can just let your listeners dial in with an access code and listen to your voice broadcast while watching your presentation online. (By the way, always find out how to mute listeners' lines so that you don't hear them talking over you while you are presenting. Or worse, have one of them put you on hold, playing music to the rest of the audience!)
There is one serious drawback to using an automated call-in system though. I'm not aware of any such system that gives you a "green room" capability. For those unfamiliar with the term, it comes from the entertainment world and indicates an offstage area where performers can prepare without being seen and heard by the audience. For a web conference, it gives the presenters and administrator(s) a chance to talk and prepare for the session without the telephone audience hearing them. Most operator-assisted meetings have this feature. The operator simply joins the two sessions when the presenters are ready to go live.
Talking to each other before the presentation ensures that everyone is present, lets you cover pre-show reminders, allows you to solve last-minute crises, gives you a chance to check relative volume levels between presenters, and generally eases tension and nerves. All presenters know exactly when you are going on the air.
It doesn't seem to this phone technology layman like a difficult feature to provide. Almost all automated conferencing systems have a way to distinguish between presenters/hosts and audience members already. Simply block the receiving line for audience members until the host punches in a code connecting them with the presenters' audio stream. Voila.
If you are a vendor that allows such a feature on your automated phone conferencing system, please let me know so I can recommend you to my clients. If you are a user of such a system, clue me in. And if you are a phone conferencing vendor looking for a way to introduce a useful differentiating feature, I humbly suggest you add this one before your competitors do.