Maybe not.
The NiCd batteries can be left on contsantly charging (as is the case with these phones) without problems, but for a NiMH battery these can only be charged for a set time before problems can arise & they can explode etc.
It all depends upon the charger base. If it is an intelligent charger/base & it detects & stops charging once full then it should be OK, but if not it can be a hazard.
I would reccomend that you put NiCd batteries back into it.
They are still available in some stores.
What size are they? are they AAA?
http://www.ardelectronics.com/ - select "batteries" then "NI-CAD" and then your type. Although you will get them elsewhere too, maybe even your local poundland.
Although NiCd batteries are being discontinued eventually, to be replaced by NiMH as they are supposidly better for all the charging side is different & not all manufacturers have switched over yet.