Lead acid & NiMH batteries are interchangeable in this phone.
3.6 volt and 4 volt batteries are interchangeable with this battery.

Sony BP-T40, Toshiba DT9000, 2300198, NiMH Battery

Outline	Model	Type	Capacity	Price	Order
	CS-BPT40CL (Generic BP-T40)	NiMH (longer life than lead acid and no requirement to keep fully charged)	3.6v 700mAh	$16.50Cdn ($13.20US)	Quantity:

The above batteries will have different model numbers on the product pages since they are after market replacement batteries which are as good or better than the originals.  The current or mAh (milliamp hour) rating of a battery is an indication of the battery capacity or how long it will last on a charge, the higher the better.

There is a $7 Cdn ($5.60US) shipping and handling charge.
90 day warranty and no questions asked 1 month return policy.

Cordless Phone Batteries Home Page