Your phone/computer should (according to the standard) know when the leap second is and apply it. NTP etc. transmit the information about leap seconds well in advance.

It is possible your device may not see a leap second, only eventually observe that it is running one second slow, and smear to correct for that.

NTP includes flags for "leap second at the end of day", so devices that check NTP time at least once a day get the information. The systems internal list of leap seconds is possibly updated before that through normal system updates.