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.
While I have no sympathy for the upper management behind this stupidity, I would like to remind people to ensure their fire is focused away from developers and other non-management staff. They generally have no say in these calls.
If you have the connections to pull their staff away into jobs at other companies, please do that instead. It may not feel like it, but it will do more long-term damage if you can extract their senior talent.
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.