They list three different vendors on the site for prescription inserts (one per region), and there are other vendors who offer them, too. The partnered US vendor had them listed at $150 but I think some of the other options are cheaper.
- If you use our client you can use our servers
- If you don't use our client, you can't use our servers, but you can use any other server
It's like, technically it's sometimes[1] OSS, but they don't care about actually being FOSS in practice. If I can't fork the software, add or remove a feature and keep using the software's other features, it hasn't hit the bare minimum to be called FOSS, IMO.
1 - Most old versions of Signal are OSS, but frequently updates are only shared after a long delay - in some cases over a year out of date, if my memory serves me.