I explored Belfry Luna/Sol and I pull the lever that rings the bell but I totally failed to understand what does that actually do. Apparently, nothing happens, at least in single players but since those are pvp areas I wonder if it triggers some kind of online events.
If you're referring to the lever at the top of the ladder in Belfry Luna[The area you access via the Pharros' Lockstone passage] then it opens the gate blocking the mist for the Gargoyle boss fight.
The second bell in Belfry Sol (the sunlight roof area) unlocks the gate at the end of the rooftop area. I confirmed this today.