I did a quick google search and couldn't find a "fg7s tellguard". Closest I could find seems to be TG-7FS Telguard cellular dialer?
Could you be a little more descriptive about what you mean by "going off hook"? If it were a landline phone, I could guess what you mean; but since there's no physical "hook" in a cellular, you're using a jargon term and jargon terms tend to be ambiguous outside the technical community that uses them.
What, exactly, are the symptoms that you're experiencing? If you're dialing the number from outside the site (home, business?) and getting a "busy" signal, we need to know that. Or whatever other symptom leads you to the conclusion that it's "going off hook".
Also, might be helpful if we knew the panel/controller on site that interfaces with the "fg7s". Sometimes a dialer (of any sort) behaves erratically because it's being told to do so.
