Unfortunately, you answered your question by self. You said:
1. every single one had a phone system that would give an audible beep when the caller was placed on hold to remind everyone that a call was still waiting
2. Cox ... say that this is not a feature on the Cisco phones they provide
In short, you have no system supporting the feature you are requesting. There's nothing you can do with it but change of system you are using. Replace Cox system by a system that will satisfy your demand.
Yes, I know it's not the answer you wished for, but phone can't support the feature that's not supported by upstream call control system.