Did not see any requirement of the sort in the fine print, but even if there were, it’s fine as long as you pick the right provider. If I had to make the occasional call it’d be still worth it. There are also providers that will keep a sim active indefinitely as long as you “purchase” one month (as little as 5€) every 1/2 years (most importantly, they do not charge you into negative credit). So basically free to operate as well.

Honestly I do it mostly to limit spam, if I did it only for privacy reasons I’d have more than two numbers but I fear one might start getting noticed by the autorities at that point :/ sms is inherently unsafe and not private.

Every sim slot has its IMEI

Other than avoiding those services as much as possible, I use a second phone number for “machine-communication”= whenever I’m not giving my phone# to a person.

I’m in the EU, I found a provider in my country that offered a prepaid sim card (pay-per-use) with no expiry date, but never use the credit on it because it’s free to receive sms. I turn it on in my dual sim phone whenever I need it.