Well again (if) they say 10-15, then it's really 10 and that's under optimal (lab) conditions. I guess at minimum the following would apply to reach that number:
1. You have a pristine cell signal always (ie: you are standing under the tower and never moving).
2. Since you are making calls your screen is never on and in the brief moments it is for calling or answering, screen brightness is set to minimum.
3. You are not using any mobile data or wifi for anything.
4. All background services (sync, location, etc.) are disabled.
Try it again under those conditions and see what you get...
I have a cell tower in my backyard and I'll be the first to tell you that nobody can ever operate in pristine conditions because even if you stand under the tower you will not connect to it. You must be a mile or so away at least to connect to it. Otherwise your phone is connecting to an even further tower.
True conditions to test the battery of your phone are:
Do you have decent cell signal?
Have you factory reset your phone to remove all downloads?
Is the normal daily use conditions set? (Medium brightness, data on, wifi off, airplane mode off, etc)
Fully deplete your battery and fully recharge without use. Do this twice and after the second full charge use your phone as normal (don't download your apps back for this, you will not get accurate data on your battery at factory settings) and also make sure you disconnect your charger promptly once your phone charges to 100%. After you have charged your phone fully the second time, use your phone to make phone calls and count how long your battery lasts. In these conditions I would guess that you get about 8 hours of battery. I believe that the marketed reports are raised a bit beyond what the labs suggest when tested with barely any use.
I began reading this forum because my voice calls default app uses 79% of my battery. I have a new Huawei P8 lite. The battery on this phone could be stronger but the phone comes with amazing battery saving software I wish other phones had. However, 79% is still excessive battery drain at the end of the day. Maybe somebody will answer the issue on this forum in the far future when phones rely on our body heat as power.
-Shadow