Grandstream Fast Busy On Outgoing Calls

I recently signed up for service and installed a Grandstream behind the router. Call quality is fine and incoming calls always work (to my knowledge). However, every so often we get a fast busy when trying to make an outgoing call. Rebooting the Grandstream clears the problem for a while, or we can usually just wait for some (as yet undetermined) period of time and the issue resolves itself. I swapped the Grandstream with another device under warranty, but the new device is exhibiting the same behavior. This is incredibly aggravating, anyone have any ideas? The only thing I can think of is that there is also a Telo for our residential service and it is the first device after the modem. Could that be causing a problem for the Grandstream? Any suggestions would be greatly appreciated!