I would like to make a conference call by selecting some contacts from my app, is it possible? also is there any limit on the number of devices in conference call? Is there any Android SDk support,any version is fine for me? Plz give some inputs.. Thanks In Advance.
You cannot manage a conference with a smart phone. You need an intermediate service that can do this for you. You can program a conference manager using CCXML. Voxeo has a good hosted platform for CCXML implementations and you can look at their documentation on how to setup conferencing. There are examples in "Learning CCXML 1.0\Multi-Party Conferencing in CCXML 1.0". You can develop and test for free on Voxeo and they only start charging you if you put it into production. Another option is Twillio.
In order to make a conference call, you need to have certain API's for functions such as HOLD, SWITCH and JOIN.
I don't think it's possible currently. Refer this thread.
Also see How to make conferencing of two calls.
来源：https://stackoverflow.com/questions/11430620/is-there-any-api-support-to-make-conference-calls-programmatically-in-android