Not to bad. Link to the description of the phone.
http://www.intomobile.com/2012/05/16/spareone-phone-lasts-15-years-single-aa-battery/
Nevertheless I am working on a 65 gram bird tracker with the target to run endless. Right now Eneloop batteries are in use. Eneloop offers a self-discharge of 25% within 3 years. If the self-discharge is linear, then after 15 years 23,7 % capacity is left. Such a AA cell offers 2400 mAh hours. After 15 years will be still 577 mAh left.
The phone promise 10 hours talk time. 10 hours talk are 5 hours transmit and 5 hours receive? I am not sure. Anyhow a state of the art GSM module discharge the battery with 230 mA during maximum power level of 2 Watt. 2400 mAh / 230 mA = 10,41 hours. So after charge the 10 hours are possible and after 15 years we have 577 mAh / 230 mA = 2,5 hours transmitting time left.
The AA batteries are public at the link as below. Just calculate yourself.
http://www.eneloop.info/products/batteries.html
Anyhow, this phone could run endless too. It just has to use energy harvesting by solar or temperature delta.