When you are fighting crime on-the-go, you can't always stop to check for available wifi. In these situations, sometimes you need a mobile phone other that your primary phone. I call these secondary mobiles "burners" because in addition to being a huge fan of Batman, I am also a huge fan of "The Wire." I call a prepaid mobile batphone a bat burner, because a batmobile is a car, and crime-fighting vehicles are beyond the scope of this website. In general, prepaid mobile minutes and text messages cost significantly more than the minutes and messaging that you get with a contract mobile service. Compared to the per-minute price for most VOIP services, prepaid mobile service is outrageously expensive. In addition, the per-minute price for prepaid mobile service is directly affected by how much money you deposit into your account at a time. Prepaid minutes also have an expiration date, and the expiration date is also affected by how many minutes you purchase at a time.
My prepaid mobile experience is limited to t-Mobile, so keep this in mind when you are budgeting for prepaid mobile service. Also prepaid service has changed a lot since I went shopping for my batburner. Also, the T-Mo Gold Rewards thing that I signed up for isn't a thing any more.
If you aren't going to be using your burner very often, or if you will mostly be using it on WiFi, then you can go with the cheapest plan available to you. In general this is about $40 per year, 10 cents per minute and 10 cents per text, with no provision for mobile data.
Prepaid mobile data is hard to find inexpensively. I have had limited success setting up a prepaid plan for calling and text on an old unlocked Nokia, and then plugging the SIM into an unlocked smart phone. That way you can use the phone on WiFi for pretty much everything, and just do calling and texts when you are on the move. You can also combine with with the mobile HotSpot feature on your "real" smart phone to use it as a Wi-Fi hotspot for your batburner. While this is handy, it is a terrible idea if you are using your batburner for privacy or security reasons.
If you will be using your batburner for more than 30 minutes per month, or for more that 500mb of mobile data per month, you will probably want a prepaid monthly plan instead. This will push your annual price up to over $100 per year at which is fine if you have the budget for it. If you don't, you may want to consider a hybrid approach where you use the prepaid mobile and some form of Internet Telephony app or device when you are not mobile.