Inside-application database design + RGs

I have “Appointments” DB in my application. It is built on “Calendar event” RG.
I also have “Clients” DB, built on “Customer” RG.
Is there a possibility to create new thing in “Appointments” and if there is a Phone number that is already in “Customer” RG, then find a row in “Customer” RG and “Make changes” to this row and add new Appointment name (list)?

CE DB C DB