What happens if more voltage hits your mobile phone charger?
Most likely nothing will happen. I can't say for sure because China... is China. 

People that design mobile phone chargers always leave margin for errors. I am not 100% sure how much the margin is, but some designers make chargers meant for 240 V with 1000 V  rated components. Some might use 400 V instead. 

It's very bad design to use 240 V rated components, because then, as you might understand, things will break if there's 300 V (error). People that are working with this most certainly know what they're doing and use good design, aka make your design at least twice as strong for what it's meant for. 

If you want to be 100% sure that nothing will happen, open up the charger and look through the components and google their ID and look in their datasheet.