For about $20, you can buy a phone that comes with 10 minutes of talk time and has its own phone number. You can use that as a temp number. If you don't buy more time for that phone, the number will go away and you can then buy time for it and get a different number.

Or, what I do, is just keep it loaded with a minimal amount of minutes and use the same number for all such registration purposes.

I started doing this when I needed a whatsapp account, but didn't want to give facebook my phone number. I just keep that phone number alive, but never use it for anything else, and never even turn the phone on.