You may want to clarify what is meant by "anonymous" reporting. In all the states I've worked in over the years, the CPS people want to know who you are so they can contact you if they need more information and, in many cases, they are required to report back to you about the outcome of your report. However, they do not reveal to the suspected abusers who made the report(s) when they do their investigation. When I worked as a hospital surveyor for my state and the Feds for a number of years, we handled complaints about hospitals the same way -- we wanted as much info as we could get about the complainant (and were required/obligated by state regulation to report back to them our findings when the investigation was concluded), but we never revealed to the hospital who had made the complaint -- that was kept strictly confidential.
Now, having said that, that doesn't mean that potential abusers (and the hospitals we investigated) couldn't possibly figure out on their own who made the report/complaint ... (Like, if the report of suspected abuse involves info that only one or two people would know). But they sure didn't hear it from CPS or from my survey team (in fact, in my agency, we used all kinds of clever tactics and ruses, "decoy" charts, etc., to do our best to keep the facility from being able to figure out who might have made the complaint).
Also, in most states, if you are a mandated reporter as a health professional, it doesn't matter whether the information came to you through work or through personal channels -- you are still a health professional even when you're off duty. In most states, mandated reporters can be charged as accessories to the abuse if the abuse is substantiated and it becomes known that a mandated reporter knew about it but didn't report it -- that's another good reason to report "on the record." There's only a slim chance that it would ever come to that (you being charged as an accessory to the abuse), but, if it did, your saying, "You remember that anonymous call you got? That was me" wouldn't help you v. much.
To me, it comes done to basic professional responsibility and accountability. If we want to be taken seriously as professionals, we need to be willing to be accountable for our actions and practice.