Using an Android phone like a SIP ATA

I would like to get a few Android phones functioning as a SIP ATA so I can make and receive calls via SIP. I realize one could use Bluetooth to pair with something like Asterisk, but I had high hopes there might be a standalone application that would transform it into an ATA. Is there something out that that will allow me to do this? Edit: Yep, searched Play Store nothing to be found unfortunately.