I can concur the BL-5C is supposed to be an 850mAh battery standard, and is smaller than the X20 battery, and will fit in the X20.
I just acquired an unloved Nokia 6230i from my gf, and I really wanted to test this.
I was hoping the X20 battery would fit the phone, because then I would have a bonus free AC fast charger (i.e. USB voltage).
The phone's battery life seems to be half what it should be, granted it is nearly a couple years old. I might get a longer life one for the phone if available, and alternate batteries, having one spare charged for the X20 on those travel occassions.
I'd like to find a newer phone that can use the X20's battery, then I'd have a flimsy excuse for interoperability and a new toy
PS: The only time I have run out of battery was going half-charged into an 8hr voice recording test of the X20, rare and one-off. The phone's voice recorder seems to be heaps better than the X20's from a 1-second test.