Separate Phone and Internet Modems

I'm in almost the same boat as you.  I have the Cisco DPQ3212.  I decided to cut out cable TV, but keep Cox phone service and upgrade to the Ultimate Internet package (500 Mbps) from the Preferred Internet package (150 Mbps).  I did this last Friday, the 15th and was told by the phone technician it would be effective on Monday, the 18th.  I specifically asked if my modem would support the speed.  He said yes, so I didn't even research it.  As of this morning, the 20th, my cable TV was still working and my Internet speed hadn't been upgraded.  I checked online for pending orders and it just showed "processing".

I called this afternoon, the 20th, to see when I could expect it to be done.  The phone technician told me he "forced it through".  I asked when my Internet speed would increase and he said something about it working after the system rebooted.  Nothing happened, so after an hour, I rebooted my modem only to see the same 150 Mbps limit.

I called again and the person I got this time told me that my cable TV was gone and that my Internet service was at the 150 Mbps level.  At this point, I'm kind of ticked off.  He was going to connect me to tech support before he realized that my modem was the problem.  He told me that I would need to rent a new one from Cox for $10 a month or I would have to buy one.  I told him I wasn't interested in renting one and asked what exactly I needed to buy.  He told me that pretty much any DOCSIS 3.1 modem would work.  I wasted a bunch of time on Amazon looking for a Cox compatible DOCSIS 3.1 modem with voice capability.  Of course, I now know you can't really buy one.

Searching the forums here I found the link to the compatible modems page.  You'd think one of the 3 people I've dealt with so far could have pointed me there, but nonetheless, I see that the only non-wifi DOCSIS 3.1 modem with voice is the Arris TM3402, which you can't buy, but only rent from Cox.  So, I have the following questions:

1.  Since Cox is required, by law, to provide a free modem for voice services, should they have to provide me with an Arris TM3402 for free?  Can they say that my current modem has voice and tell me to pound sand and either rent or buy a second compatible modem?  I've seen a reply from a moderator in another post indicating that you can simply take your old modem to Cox and get it swapped out.  However, the last person I spoke with didn't give that to me as an option.

2.  If I have to buy or rent a second modem, is it as simple as adding a splitter to my cable line and running one coax cable to the DOCSIS 3.1 modem and one to the Cisco DPQ3212 and giving Cox the MAC and serial number of the new DOCSIS 3.1?

I may create my own post in the Internet forum.  Thanks for any help anyone can provide.