We have a ClearOne Max conference phone at work that has been in need of a new battery for a while. When we do get new batteries for it, they never seem to last as long as they should and always need to be ordered in.
Solution: Use an RC battery pack.
The battery in the conference unit is a 7.2V 1800mAh NiMH battery pack. The RC battery pack is 3000mAH so we get much longer out of it on a single charge.
We’re still able to use the normal charger that came with the phone too.

Sure it might be big and ugly (I’ve hot glued it to the phone) but it works amazingly well.