In muggle mythology, there are several mentions of magical instruments, bound to attract help for whoever yields them. The closest reference I could find was from the book Lord of the Rings (I will lend you it, if you are interested. Worry not; it’s a fantasy novel, no muggle artifacts to be found): The Horn of Gondor. Here is what it says about it:
“If it be blown at need anywhere within the bounds of Gondor, as the realm was of old, its voice will not pass unheeded.”
This has given me an idea—something alike to the protean charm, a sound only the members of whatever group is formed will be able to listen and recognize as a call for help. What do you think?