You can draft some policy regarding the usage of mobiles, but don't think you are some person in a fair handing out pamphlets to visitors. If you think of handing a memo to an employee, it's unfair, and your policy should state that the usage of mobiles inside office premises is banned. If anyone receives a call, they can go out and speak. Don't invade the privacy of employees. First, can you stop the top management from taking mobile calls inside their air-conditioned cabins?
If you are so strong, why not install a mobile jammer inside the office so that no one can use their mobile? Though your policy idea seems okay, the memo idea seems not okay. I hope you can tell me what trouble you are facing regarding mobiles. In my company, everyone uses mobiles and speaks, but it does not harm or affect other staff. What if official discussions are done on a mobile? How can you issue a memo for that? Please clarify for me.