I use Bria, and it's decent. It supports push notifications for calls, which is necessary on iOS. The app costs money, but comes with excellent support if you run into issues. There are free and open source apps, and the quality of these are hit and miss.

The push notification necessitates the app sharing SIP credentials with Counterpath (the company that makes Bria), so it's good to have some fraud protection for the extension in Asterisk in the event that Counterpath gets breached. They are a publicly traded company, so hopefully they have decent security.

The one built into Android is basic but functional. It's a bit hidden in the user interface but here's how to configure it: https://support.google.com/phoneapp/answer/2811843

It's not useful over the open Internet but on a local WLAN or over a VPN call should work fine. Unless you use a damaged phone book storing SIP URLs for your regular contacts should work too. No messaging or fancy codecs though.