The feature is called Mult-Network Backup Add-on. I pay $7.50 a month for the feature. Say, I am in a bad cell area and my primary network Warp - (Verizon) doesn't have signal, it will automatically switch to Dark Star (AT&T) The add-on is essentially a second line with a separate number. If my family needed to get a hold of me (or someone else) and I was in a bad area and the phone switched to AT&T, the people calling me would need to know my Dark Star number or wait until I was back in cell range for Warp. Hope that wasn't confusing.
You can try US Mobile's service free for 30 days.
Without the add-on I pay $25 / month for unlimited talk, text, and I believe 50GB of data.