Here's a thought -- may not apply to your KSU, but some of the Pan analogs have a programming section that's something like "allow paralleled telephone". If that's enabled, you can hook up an SLT in parallel with the T/R of the key phone, and both will work, though not simultaneously. This is NOT like the digital KSU's; both phones will be the same extension.
If the SLT is alone on the extension, it should work, but note that the default action on pickup is to go to Intercom. Trying to dial an outside number will be invalid for the intercom and you'll get the Reorder Tone, which does sound a lot like a busy signal. Be sure the extension in question is programmed to allow CO calls, and then try dialing 9 when you pick up, and see if it grabs a CO and gives you a second dial tone.
Good luck!
