VOIP is a bit of big world, with quite a few contenders and different services.
If you are looking for VOIP on your smart phone you need two things. You need an client "app" that can be installed on your phone and connect to the voip (called SIP client), and you need a voip "service".
Some voip providers provide the app too, their own app. There are also a few third party client apps.
The thing about VOIP is, all of them are telecom companies and all voip communications ultimately go through the telephone network. This means that every conversation you have is no different from a regular phone call and is recorded and transcribed secretly by the governments of the caller and recipient. Even if your "provider" is safe (and there is no such a thing), the recipient's is not. Same for your SMSs. All telecom companies in north america are bound by KYC laws and so have a firm identity tracer on whatever account or number you have from them. Which makes them not that different from a regular phone provider. Although some voip services are a bit better and more respectful than normal telecom companies.
Another point to consider, many voip providers spy on you as well and sell your data. They build relationship maps from your calls and numbers and sell that. Not all of them do. So not only is the main telecom company (AT&T, verizon, etc) building this data (to be given to government only), the voip company does this too (to be sold to anyone, not just gov). Consider that the provided apps (one's that provide their own app) are not made with privacy in mind and in fact many of them try to spy on your contact lists just like whatsapp does. They encourage you to give access to your contact list "for your convenience" with the aim of stealing it. Major voip services can do this, just like whatsapp.
There is one company I know, jmp.CHAT which provides great policy, but is expensive relative to other more mainstream voip options. There are more companies, but I can't recall their names now.
Despite the cons there are also benefits to a voip line. For example you are not location tracked all the time, where as on a sim number, you are, through antenna triangulation. On a good voip service and sip client you can have more privacy than a regular phone line. Although all conversations are still "intercepted" and transcribed by the government, they may have a harder time acquiring your identity and location.
Have a look at hushed in comparison. Good unlimited price, but pretty bad privacy.
So to get good solution you need both a good service AND a good client app.
I'm after the same thing. If you find a good solution please let us know too.