Step 3: unfuck the SIP settings, then email both HR and their supervisor to throw them under the bus. Also covers your ass for step 4.
Step 4: Route the manager’s calls to a disconnected number. When they come knocking about their phone not working, tell them, “No, you should be able to dial out, unless someone changed the SIP trunk settings and didn’t tell me.”
Assuming you already have the IP phones, you need two things. A PBX server (for the VoIP stuff), and a SIP trunk with a block of external phone numbers.
Start with the PBX server software, there’s several free/open-source implementations. Once you’re comfortable with it and have internal calling good to go, then you can spend on the SIP trunk and number blocks.
If you have admin portal access to write transport rules, I recommend writing a rule that has the server reply with a 550 5.7.1 Delivery Refused error. Trigger it based on the keyphrases “top of Google search” and “affordable SEO” to start with.
Computer monitors should work too, and are more readily available. Just dig through the business oriented monitors and ignore the gaming ones, as cable providers aren’t really going to have anything that can take advantage of >60 fps display rates.
SIP providers usually sell numbers in contiguous series for businesses. For example, if your company buys a block of 50 numbers, the SIP provider then allocates XXX-5100 to XXX-5150.
But since you’re keeping this strictly internal, you don’t have to worry about that.