I can confirm that the few times our phone has used data by accident, some funds have deducted. Never without data use though. It does seem expensive, so it may be a good idea to be careful until the cost per MB or GB can be established for sure.
To eliminate that, I have placed a hard cap on that sim of 0.
Thanks, chosen & tech-stitch, for your observations about data overages.
That would jive with why I haven't seen any charge-- my FreeUP SIMs are in dual-SIM phones, with another SIM designated for data use.
I also use Google Voice in forwarding mode to my carrier number-- that might mean that with careful attention to blocking data access from other apps, I could manage to use the current no-data FreeUP plan in a single SIM phone, with the needed minimal data being charged against my balance.