I *think* you need to take a copy too. Unless they *help* you at the counter for a small tip without paperwork. Or if its true you don't need it anymore..
The number is permanent and attached to the sim card, which you can move phone to phone keeping the number. You can buy minutes cards at Oxxo and places and put more pesos on the card. Or, what I find easier is to set up your phone on the telcel website. Then you can add pesos by US credit card whenever and check your balance. It is in Spanish, but it is easy to figure out.
What does happen is without use or adding minutes, it gets suspended after a couple months or so. Then you need to add a few pesos to get it working again. Usually before a trip I turn the phones on here and let them connect. They will roam on the ATT here. Then I hit the online account and throw 50 or 100 pesos on them. Then I get a text message from Telcel acknowledging the recharge which lets me know the phone is live again. I guess if you wanted you could fire it up and send a txt message while roaming to keep it active too. Before I set them up online, I would always have to wait until I got to the ferry pier to run in and get a new minutes card to make it active again. I would hand the girl pesos and my phone and tip her for charging it for me. My wife speaks Spanish and has a hard time understanding the fast Spanish on the phone prompts. I STILL don't think we have voicemail set up on any of our phones. OR at least I cant get to it.... So if you call, don't leave a msg, just text me.