It might be difficult to stay anonymous in this situation if you mean stay anonymous to the police. Even if someone else calls for you or if you call using *67 the police will probably want to talk to you to find out if you saw anything. Usually, when you call the police they press you for your name and other contact info. Plus, I assume the police can figure out the number calling if they need to for an investigation even if you use *67, but I’m not sure.
If you are afraid the criminals will find out you called, I don’t know the laws around confidentiality for that. I would hope the police can’t release that info unless you say it is ok.
I think I would call the police, but that’s not advice, it’s just what I would do.