This is not smart at all, cou nan burn telephone line wires, also you need voltage stabilization to around 13,8V for charging one 12V battery. Phone lines cant give you lots of current, usually small devices which use phone line to charge their batteries use pulsed charging method.
If you leave in some country with modern telecom, they can have automatic protection and monitoring systems for each subscriber. Protection system trigger is when current is increased over some level, or current is drained on longer time, example over 30mA, and system automaticly react and disable that phone line, after that telecom operator need to investigate where is trouble, and manually enable subscriber line. Subrscriber line can be permanently disable if usage of that line is not in telecom recommendation for usage.
Better buy small solar panel.