I just completed phone reservations using the Firenze card, for Uffizi and Accademia, here's my experience in case it helps anyone.
After reading about the Firenze card with possible 48hr extension we decided it would be a good deal for us (we'll be in Florence for 7 nights). I bought 2 cards through their website (you must enter name, DOB and email for each card) and received vouchers via email. We each downloaded the app to our phones and followed the instructions to enter the voucher for each person. That basically loads your personal digital card to your app. They warn not to delete the app after you do that because you'll lose the card.
The whole process was clear and quick, much easier than expected. I wasn't sure if I needed to purchase the cards before making our reservations, but it turns out I didn't need to.
This morning I called the Firenze Museii call center provided on the website (+39 055 294883) and followed the instructions, press 1 for English, then 3 for reserving times with the Firenze card, then 1 for accepting privacy policy. I was on hold for less than 10 minutes (3rd in line at the start, with recorded updates on position in line). The operator came on, started in Italian but I just said I wanted to make reservations for Uffizi and Accadamia. She asked for dates and times, I provided them based on what I had looked up as available from the B-ticket sites. She took my name, and for each museum reservation she gave me a confirmation number (one number for 2 people together). She didn't ask me for my Firenze card number. She said I just go to the reservation desk at each museum and present the confirmation number along with the Firenze card.
Again, all very easy and less complicated than I expected. I made the phone call using Skype (total 11 minutes). I have Microsoft Office 365 and didn't realize there's a 60 minute subscription for skype calls.
I'm providing this long winded description because there were a few things I worried about before doing it that I thought might help others:
You DON'T actually need to buy your Firenze card before calling for the reservation. I'm guessing that reservation is only good if you show up with the Firenze card, but to make the phone call and book it, I didn't have to provide the card and the reservation is FREE. Remember your card isn't activated until you enter the first museum (72 hour clock starts then)
Just before making the call I looked up the available time slots on the B ticket site and was ready to give the exact dates and times I wanted when she asked for them. I don't know if I could have just asked her what was available, or if I asked for a time slot that wasn't listed as available whether they have some leeway or extra spaces reserved for Firenze card holders.
I believe calling this call center is the only way to make the reservation using the Firenze card before arrival in Florence. According to the FAQs on the Firenze card website there are several places you can go in person to try to reserve with the card but we didn't want to risk getting our preferred time slots. I don't think this reservation is the same thing as booking online from B-ticket site and selecting free ticket with 4euro fee. I think that is just for the groups of people that are allowed free admission to the museums.
