I have to ask:

What possible legitimate reason could you have for this functionality?

My company operates a Rightfax server with a full t1 line for inbound faxing for loan apps for credit approval.  On several occasions either the line has gone down or one of the toll free numbers (we have over 100 toll free numbers on this system) has gotten screwed up and it usually takes the users/admin staff about an hour to notice.  This can lead to several faxes being lost due to line or number issues resulting in possible loss of income on origination and servicing over the life of the loan.  

Our carrier says that they can't pro-actively monitor voice lines or toll free numbers pointed at them; stating that testing is the only way to see if there is a problem.  This testing is strictly reactive in nature as one must call the support center and initiate the testing.

why not just set up another box on another line running something like hylafax and run a script to periodically send faxs to the original machine and monitor the output log for problem numbers.
This is going to up the costs of the line I would think and will tie-up your lines for short periods but I dont see how else you can do it.
For some good starting points on scripting it all yourself drop by

http://www.faqs.org/faqs/fax-faq/

This sounds like a job for a custom agent that you configure to "Poll" each fax line on a schedule and then reports back to unicenter on it's status.  I've written a couple of agents already and they really aren't that hard to get running, they just take some planning.

I'm not sure but it also sounds like a good candidate for the SLA option.  I don't quite think that SLA would dial each line, but if you had the agent in place, SLA could watch the agent.

let me know if you want to go the route of a custom agent.  I'd be willing to help out.

Martin

If the sky is blue and the Sun is yellow....  Why isn't the air Green?