If you have a class of 25–30 students, divide them into two groups and work with them online at different times.
Next, you need to set up some chat rules which should cover such issues as turn-taking, allowable language, and the participation of all members (not just a few vocal ones).
While this list seems to exclude many students, I have personally seen voice chats work in very remote locations with extremely slow dial-up internet connections.
A headset can easily be purchased for less than $20 and today sound cards are not high-priced items either.
Other teachers assign a reading passage to be discussed.