I have a smartphone and I keep getting messages that I am over my data usage. I am guessing I am downloading podcasts and using the podcast app that is using the cellular data,
I can vouch for this problem myself. The last two months I have gone over my data usage. When you want to use a WiFi signal you have to make sure your phone has all of the correct selections. This varies by phone and carrier.
For Podcasts you can specifically set your phone to only use WiFi.
Look in your phones settings and review the WiFi, cellular, Airplane Mode, and Hotspot settings. Also review your app settings. Make sure they all are set appropriately to use WiFi. One way I found to make sure I had things set for WiFi was to turn on the Airplane Mode setting review the settings mentioned, and then turn on the WiFi setting.
Remember all phones and services may have different settings but it is worth the time to review them so you don't pay for extra service you don't need to pay for.