The NiMH battery will power the phone just fine, once it's charged. But you need to have a charger designed for NiMH batteries. Some Ni-Cad chargers also are designed to charge NiMH batteries, but some are not. (I assume there is a difference in charging current or something.) Don't use NiMH batteries in a charger designed only for Ni-Cads. Since you will undoubtedly be charging the batteries in the phone, not swapping them out into a charger, you shouldn't use NiMH batteries in a phone that says to only use Ni-Cads.
The only exception I would make to this is if the phone vendor sells a NiMH replacement battery specifically intended to be used in the phone in question. And I would go with the vendor's battery, in that case.
Michael D. Sullivan Bethesda, MD (USA) (Replace "example.invalid" with "com" in my address.)