For years I’ve used CSipSimple but lately it stopped working properly on my Note 2 after the last system update.
Looking around I found Zoiper, another free client, which seems to work.
I’ll update this post as and when I try out other clients.
Integrates well with Android dialler. Many calls, usually good quality. Worked very well up to Android 4.3, since then dodgy. Tried the nightly builds but not much better. Free and no ads. Worked with DrayTel.
No dialler integration but can read address book. 2 calls so far, one great quality the other shoddy. Free and no ads. Works with DrayTel.