The question is when and how often they resynchronize over the Internet. I doubt that they directly depend on the frequency in the power supply as both a PC's mainboard and the phone are not directly connected to the AC power supply system. Both of them have AC-DC converters, phones (and laptops) can even be entirely independent of the grid as they of course have batteries. So if your problem is related to the frequency in the grid, then I assume that's due some other device sending your devices time data. I'm not sure whether infrastructure in the mobile network relies on the frequency to provide time information.