A phonecard is a small plastic card you buy with call-time already on it, then push into a public telephone instead of coins. For years this was how you called from a payphone in a station or on the street. Mobile phones have made phonecards much rarer, so younger students may never have held one; older people still know the object at once. It is an ordinary, slightly old-fashioned object-word — not slang, not formal.

電話卡（公用電話儲值卡，不是手機 SIM 卡）。