@Leppattep39. What kind of phone card you mean? I suppose it is the so-called SIM card, which you need to insert into the mobile phone. There are two possibilities. If you have a mobile subscription (A(bo)), then you can cancel or pause it, see here (herein you'll find another contact link):
https://www.swisscom.ch/en/residential/help/data-and-bills/cancelling-on-hold.html
If you have a prepaid card (not a subscription) you can simply use up the credit and then destroy the SIM card. A prepaid number automatically becomes inactive if it is not used for more than 12 months.
Of course you also can call the customer service, phone number 0800 800800 (it is for free).