SIP client VoIP app application software for Android smartphones. Partially free. Premium features available for a cost. Many versions and variations.
Review
This uncoordinated disaster mess of apps, versions, upgrades, and addons is poorly coordinated to where no one can understand what they need to pay for, upgrade, or even install in the first place. Once installed the user is nagged to upgrade and pay for addons when the app itself is unreliable and wont maintain registration status to the SIP provider.
I would strongly discourage anyone from getting stuck in this tar pit of poorly documented crap-ware. The ZoIPer devs do not have it together.