Textnow is fine. It's not 2-3 days for calls, but more like 30 days, to keep your same number and keep it active. You can actually "subscribe" for $5 a YEAR to keep number active without calls.
TextNow uses only their app to do texting or calling, which seems indeed to be "unlimited." With their SIM in a smart phone, you can use their app (and their app only) on the TMobile network and it works for SMS, phone calls, etc. On Android, you also get "alerts/notifications" for other apps on your phone, but you cannot use them, without WiFi or upgrading to a data plan.
The SIM does work on an old Android phone I have, and works well.
Ads are not in calls, or via text, but inline with texts in the app, but easy to look over. There are banner ads in the app, and inline with your texts. I haven't found them a bother.
Don't expect any support. While they do have chat support available until around 2:30 EST each day, I've never been able to access it, and from Reddit, those that do seem to rarely get anything more than their very most basic queries answered.
I have one line which works fine. The 2nd line, I left expire, and now, even with a new number and SIM, I can't seem to get it to work, but from what I've read, I'm just frustrating myself.
App gets updated on Android all the time, so they must be doing something!
I don't work for or get paid to shill for them. Just letting you know my real-world experience over the last 3-6 months or so.