How exactly are you using iMessage with your VoIP number? I thought it requires a valid SIM card to register the number. Also, aren't SMS, MMS media, and MMS groups super finicky with VoIP services?
I've been wanting to buy a sensor watch for a while now. I'm actually really glad I waited to back this now!
I would love an SPL meter on my watch with weighting filters! Don't know much about piezo buzzers but I wonder if it could be used as a microphone for this purpose.