Question about appropriate documents to submit as proof of new address abroad

Hello,

I have already gone through the FAQ about extraordinary contract termination due to moving abroad (https://hilfe.o2online.de/english-o2-community-mobile-43/extraordinary-cancellation-because-of-moving-abroad-458522), but had a more particular question about my situation:

I will have my deregistration certificate ready when I send the letter for cancellation, but wanted more information on the options available to me as proof of my new address. I will be moving to my family’s house abroad, so I will not have a formal landlord or tenancy contract, and my new job will not start until a few weeks after I have moved back. In this case, what could I use as proof to show that I will be moving there? Would a letter from my workplace confirming that I will be joining, and which mentions my address in that country be adequate?

On a related note: when making the final payment after contract termination, is the “personal customer number” the same as my phone number, or is that another number I should keep a note of?

Thank You.