Callcentric is a good, cheap, VOIP provider, with a lot of features that offer very granular account and robocall/telemarketer controls. It works with a variety of mobile SIP apps, as well as home phones. You can even add a basic SMS plan (sort of in beta, from what I understand…haven’t tried that). I’ve used it for my residential number for several years. I think they’re based in New York. But there are many good providers, I’m sure.
Linphone open-source SIP client is easy to set up and it works well on Android in conjunction with a VOIP provider. You can also add a free SIP account from Linphone itself that works with usernames in place of phone numbers. That’s in addition to any VOIP provider’s phone account and number that you add to it yourself.
P.S. The native dialer in Android can also incorporate SIP accounts. Open the dialer and check in the settings for Calling Accounts.
L5 will be getting SIP functionality in its dialer, also: VoLTE support progress