PowerPivot/DAX - comparison of one column to another column

Hi experts, I currently have a PowerPivot dashboard where I use measures to concatenate and return the names of clients who have received certains types of service:

Call Recipients
=calculate(CONCATENATEX(values(ServiceHistory[Contact Name]), ServiceHistory[Contact Name], “, “), ServiceHistory[Service Type]=“Call”)

Meeting Recipients
=calculate(CONCATENATEX(values(ServiceHistory[Contact Name]), ServiceHistory[Contact Name], “, “), ServiceHistory[Service Type]=“Meeting”)

I'm trying to figure out how to set up a measure(s) to return the names of clients who have receive one type of service (Call), but have not received another type (Meeting). Any idea how this can be done?

Thanks in advance for any advice!