I did the same thing about 3 yrs ago - the Sony cordless phone had already been thru 2 rechargeable pacs and got to where you'd get the low voltage beep in less than 5 min talk time. That phone uses full size AAs and I made a pac using 3 2300mah PowerX batts. 3yrs later, it will let the wife talk for over an hr and still not beep. I really think they put cheapiest cells in them possible so you'll have to periodically replace them.

It saved me from buying another phone!