Cell phones are increasingly common for even young children, in divorce. I understand you have concerns, but you should be measured and judicious in your response. If you are in a contested custody situation, the court may not look favorably on you "interfering" with your daughter's ability to contact her father on a phone he provided.
But certainly, having the phone in your home gives you the ability to check it, to see who she's talking to, when and how long she's talking, etc. If there are problems, you're capable of physically separating her from it at bedtime, during school hours or at other times it's inappropriate for her to use it (dinnertime, church...) Also, most phones have a way to block specific callers, if you need to.
Does your ex feel there are problems with him being able to reach your daughter, or being able to talk for reasonable periods of time? Or do you think he was merely indulging her with a "new toy" she liked? OR do you think he was mostly trying to needle you, to point out you're not in control of everything? If so, needle him back by not getting bent out of shape about it. It's common for NCPs to feel like the CP has total control and to resent it and feel passive-aggressive about it. Is it necessary to "win" this particular battle?
My husband gave his son a cell at age 6. He got him a Firefly phone, which can only call numbers parents program in, online. Unfortunately, no one makes phones like that anymore, to my knowledge. In his case, there was a legitimate problem with phone contact - i.e., his ex allowed none. She also confiscated the cell phone. She also lost custody 2 years later and one reason cited in the 30-page ruling was her refusal to let DSS use the cell phone his Dad provided. Curiously, ever since the custody change, she will only use DSS's cell phone, to contact him. It appears that it really is nicer, more flexible and feels more private, for the NCP.