Had to laugh at this - the client is wanting leaflets to further some purpose of his own. The phone number is a MINOR detail?
The phone number has already been provided to them - correctly. Presumably the proof process is for the client to check they are happy with design elements such as the layout, colours, print quality etc.
They shouldn't need to check things like a telephone number they've ALREADY provided in correct form - thats for the printers in house quality assurance processes.