I don't know if it was to this group, or not, but I recently posted a remark
averring that all third-party lithium-ion batteries -- even from well-known
companies -- were junk, and a waste of money. (They are.)

I can't speak for nicad or NiMH battery packs, as I've never bought
third-party replacements. (OEM replacements have always been first-rate.)
But the prices companies want for replacements are often insane.

I've never seen a cordless phone with unwired ("loose") cells that could be
easily interchanged. Part of the reason might be legitimate safety concerns,
* but most of it is simple greed. It appears that manufacturers insist on
selling a battery pack they can profit from down the line.

You might consider tack-soldering the old leads to new cells. This can be
tricky, as you don't want to overheat the cells.

* I have a thin Sony Discman that takes two AA cells. There's nothing to
prevent you from recharging any AA cells you put in the unit, and I've
successfully recharged other brands.