You should be able to use VZW for this, or any other, VoIP service. The key is whether the tower you are using at the moment will afford you the 80kbps in each direction (64kbps payload + 16kbps overhead) that you need to support a conversation. So, as with any other ISP, you can do a speed test to check your odds of success.
When I had VZW's 1xRTT service about 5 years ago there was no cap, so I didn't care. I did have a customer in a western Asian country who had a cap, and I did the analysis for him at the time, but I haven't thought of this in quite a while.
As I remember, and if you quote me I'll deny it, a G.711μ call consumes 1MB per minute of conversation. I don't remember if that is in each direction (down and up) or in both directions combined. I also don't remember how much is consumed per hour of idle time.
At the time I took the measurements it was by "reading the meters" on a Sipura ATA before and after a call and before and after a period of being idle. It's too late tonight to run the test again, but I'll try to remember soon and report the results.
Choppy voice in the other guy's ear is an indication that Sprint is not giving you quite enough upstream bandwidth. You need 80kbps in each direction, and you might be getting 56-64kbps upstream; enough for the call to be intelligible even with the dropped packets, but not enough to be clear and not so little as to make the call drop.