Develop a Mac/Android Application

We are a small real estate office in NY. We have customers that want to see apartments. They email us to schedule an appointment. I want to develop a simple app (instead of putting the appointments on Google Calendar or Google Docs sheet and letting our agents check there for appointment times etc) that a large bubble will pop up on their phone letting them know about the appointment (not on text or email). This bubble shoud take up half the screen. They will see the customers name, the appointment time, the customers budget, and time of meeting. Then our agents can click Accept or Decline for the appointment. This has to work for Android and iphone.