Some members with blackjack experience have stated that they do not hold a charge very well, just as Casey has stated.
I think there is a blackjack out there and it was hidden for a reason. I also believe it has a different account and phone number. Casey discussed how she used one SIM card in both phones and that could be because she was transferring her phone book from her old phone to the new one. I know this isn't easy to explain but it seems to be the most likely to me.
2 Accounts
2 Phone Numbers
2 SIM Cards
She stated in the 400 page discovery that she had the blackjack for work. You don't get a second phone with the same number for work. It's just not feasible.
Hypothetically... and IMOO
She has a cell phone. She has her little phonebook of friends' numbers saved to this phone's SIM card.
She gets a 2nd phone, different number and account for work.
She wants to put her old phonebook of numbers on the new phone so she takes her SIM card from her original phone with phonebook in tact and puts it in her new blackjack and saves that phonebook to her new blackjack.
Voila, she now has the same phonebook now on both phones. The only difference is, the phonebook on her original phone is stored on that SIM card, where as, the blackjack only has those numbers stored on the phone, since only one SIM card can be in a phone at one time. At that point, she very well could have went a step further and saved the numbers from the blackjack to the blackjack SIM card.
This is why she might know right off hand who's numbers were stored on the blackjack and who's were stored on SIM cards.
Whatever Casey was doing for work or why she had this blackjack, I don't believe there will be anything on it to prove Zanny's existence. I think it may provide more information as to what she was doing for work though.
And as always, this is my opinion.