I have an old iPhone with a broken screen that I threw an $8/month eSIM onto so that it has its own phone number, that I just keep plugged in with the screen off, on Wifi, in a drawer. It hosts a number of things for me, most importantly bridges for WhatsApp and iMessage. So I can actually give things like Clawdbot their own phone number, their own AppleID, etc. Then I just add them as a contact on my real phone, and voila.

For iMessage I don’t think you actually need a second phone number, you can just make a second iCloud account with the same phone number.

I heard it costs $15 for just a few minutes of usage though