the milliamp hours don't matter. Your phone will draw what it needs and not fry because the potential current is greater from one battery. In theory, the 900maH will last 50% longer than the 600. About the only thing missing is the charge rate of the battery. There are quick charging types that do better with charging circuits that can supply more current. If you can determine the capacity in milliamp hours of the one you're replacing, I'd try to match that. It may be on the label. Some of these phone batteries are several cells in plastic shrink and have info on them. Otherwise, google just might find your phone and the information you need.
I know squat about electric crap.
Trying to locate a battery replacement for an old phone I have. Doing a search comes up with two types...both Ni-Cad and both 3.6V; however, one is 600maH and one is 900maH
What's the difference, and will the incorrect one crap out the phone? How do I determine which one I need if the manual isn't available anymore? The phone itself doesn't have a battery in it to look at.