At this bookstore cafe, some people are talking too loudly on their phones and disturbing other costumers and holding up lines.

I need help on a solution explaining 'where and how' the notice will be distributed (its only for the employees) and then a policy that will be displayed throughout the store. (the policy is like a banner/sign.)

thank you

The store needs a policy about noise levels. Employees need to be trained in how to approach a customer who is too loud, asking him/her to be quieter. Signs might ask customers to keep noise levels down, QUIET, or something to encourage lower voices. Use your imagination. Keep in mind that what is too loud for one customer might be perfectly acceptable for another. What is "too loud"?