On the Android platform, there are several popular free, open source SIP applications including Lumicall and CSipSimple and the popular XMPP client Conversations.

All the leading free/open source Android apps can be downloaded from the F-Droid app store or the Google Play app store.

Users of the Apple iPhone have reported success with the iOS version of Linphone ( iTunes download link). A particular challenge for iPhone users is that apps can't keep background connections to arbitrary SIP servers open for receiving incoming calls. This is a restriction that Apple has imposed for all apps. iPhone users may also want to consider a proprietary app such as Bria for iPhone.