Yes! Anyone can report a crime anonymously. However, the complaint filed must be true and the facts and circumstances need to be clearly stated. The alleged actions must account for actionable wrong and must be verifiable. One can file an anonymous complaint through a phone call, emails, SMS, letter, etc.
However, the person rendering information doesn't need to know every particular case. Thus, FIR is a crucial process to communicate with the Police about the commission of a cognizable offense.