If you share space with other employees, when making phone calls, you should make sure that you Speak only loud enough to be heard on the phone . Option C is correct.
When talking on the phone, we usually tend to shout because our volume increases if we’re not sure the other person can hear us. At work, this can be very annoying for those who surround you, the wont want to listen to your phone calls. So the rule is to “Keep your voice down"and stay clear on the phone too.