Sam is correct; a MappedIP (MIP) is better suited for tasks such as this:
set interface "ethernet0/0" mip public.ip.add.ress host poly.com.add.ress netmask 255.255.255.255
Note that the netmask is a /32, since this is a one-to-one binding. The interface in question should be your external, or "Untrust" interface.
After adding the MIP, simply add appropriate policies:
set policy from Untrust to Trust AddressGroupOfAllowedExternalHosts "MIP(public.ip.add.ress)" ServiceGroupForPolycom permit log count