If you want to switch carriers (unless you’re on Sprint or a Sprint-related service), Sprint has your back. If you sign up for a 3 line plan (with all lines being phones, not tablets or hotspots), you will be able to get $400 straight from Sprint. As long as you’re not leaving a carrier mid-contract ($400 isn’t enough for termination fees), this is an easy $400. If you can deal with Sprint, that is. They aren’t so great in many areas.
There are a few more restrictions, like the necessity to port one line from a different carrier, having to get a qualifying device, and you have to keep them active for at least 60 days. Looks like Sprint is trying very hard to get customers.