calling_card
1. [ noun ] a distinguishing characteristic or behavior
Examples:

"bombs are a terrorist's calling cards"

Related terms: peculiarity
2. [ noun ] (finance) a card that is used instead of cash to make telephone calls
Synonyms: phone_card
Related terms: credit_card
3. [ noun ] a written greeting that is left to indicate that you have visited
Synonyms: card visiting_card
Related terms: greeting