Allow "SYSTEM" user name access to modems

Hey,
We've got a service that starts a bunch of executables, one of which needs
to access the modem to dial out, but when it's started as a "SYSTEM" process
it's not allowed to do anything with them. When I go into the services screen
and go into the properties for the service and go to the "log on" tab and
change it from "local system account" to administrator, it runs perfectly
fine. I think this has something to do with the way Vista handles services.
Is there a place where I can change the permissions on the hardware to allow
the local system account to access them so I don't have to change it from
system to administrator? Anything to this effect would be perfectly fine, as
long as it enables "SYSTEM" to access the modems. Thanks in advance.