Is it possible to send the Password reset email via SMS instead of email? I'm signing users up using their mobiles and a 6 digit pin + a randomly generated email.
Not natively, but there are some SMS related plugins for Craft 3 that might be useful.
https://github.com/bertoost/Craft-3-Mailjet
https://github.com/bertoost/Craft-3-Messagebird
Otherwise, you're looking at a custom developed plugin specific to your needs.