Thank you Paul! I ended up with the following and it calculates correctly:
=Sum(IIf([Account_Group]="MHTL" And [Equipment_Type] Not Like "POTS*" And [Service_Type]="Install",1,(IIf([Account_Group]="MHTL" And [Equipment_Type] Not Like "POTS*" And [Service_Type]="Disconnect",-1,0))))