What are your strategies for keeping people engaged during conference calls?

Ensure small jokes in between , The average user tends to get bored in a short period of time ,Humour can bring the concentration back and keep the people involved . 

Also make sure some questions are asked specifically (sometimes specific people , make sure they are not offended :) ) rather than very generic questions . Bad thing about generic question is sometimes silence can be taken for an answer. Questions like Does anybody have any doubts ? , Shall we proceed further ? . 

One of the most important things to remember before any phone meeting is to be prepared and organized. This step is crucial if you want to accomplish the items on your agenda and keep your participants engaged throughout the meeting. Prepare ahead of time for the topics you want to discuss. Create an agenda or an outline of the call for yourself and your attendees. Also, prepare as much material as possible before the meeting so that you will not waste time on useless topics 

Hahahaha, a practical problem but if we look deeply its not possible to maintain for a long time, rest the answers given by Mr. Tihab and Mr. Chinniah are well fabricated.

All answers are good to accept....I follow them all....thank you

- Send your agenda Prior to the conference call showing what you expect from each one.

- Avoid long discussions between two persons

- If you notice someone is still, ask him his opinion

- Use Ice-Breakers

By keeping workshop turn on.

All answers are respectively good.

Agreed with both answers given by Mr.: Socrates  & Mr.:Mohammed saei too

I think all answer covers the idea. 

agree with mr thiab and mr. Fouad. 

Lot of people are invited without having a real role

So

Frankly i don't' care who is not engaged as long as I am talking to the key attendees

While having multiple people logged on, topics of common interest for discussion would keep all engaged or else limit it to people who are concerned.

Make it a true2-way communication thing; speak and listen, relate to what is being said, focus on things which are of common interest,  show and demonstrate the value/benefit of what you are talking about