I understand why they make it possible to move a number to a new SIM, but I really wish you had an option to force a notification and delay the transfer for a number off days. Even a three day delay would be enough. You'd put in the request and they'd send a notification via SMS/call and email, and then the transfer wouldn't happen for 72 hours.
I would gladly deal with the potential inconvenience of not having access to my phone for a few days if it meant that it would make it harder to transfer my number to a different SIM. I don't think this should be mandatory, but I'd like the option.
It's just WAY too easy to call a cell provider and have them transfer your number to a new SIM and all the security measures that they use are easily defeated. Once you have someone's social security number you can spend 99¢ on a public records dump and get enough information to convince just about any customer service person to do whatever you want.