You learn something new everyday, and here's today's entry:

Polycom OBi phones do not appear to register with Ringcentral period. However, we found they can be flashed back to UC firmware following the instructions on this website.

Once our VVX150 phone was back on UC firmware it auto-provisioned to Ring Central.

I wasn't' able to find the instructions at the link you posted but I did find in on the .CA polycom website. Here's the instructions in case they kill that link too:

UC SOFTWARE TO OBI EDITION SOFTWARE
When you identified that your VVX x50 desktop phone runs the Polycom UC Software and you need it to have it run OBi Edition Software, you can follow these steps below:
Power-up the phone and have it connected to the internet.
Determine the IP Address of your phone by hold-pressing 1, 4 and 7 simultaneously, or go to Settings > Status > Network > TCP/IP Parameters by pressing the Menu Button.
Use the IP address to access the local web page of the phone. (Default password is 456 and use HTTPS).
Head over to Utilities \ Software Upgrade page and select Custom Server for the Server Type
:

Paste your upgrade server URL on the “Custom server address” field. For OBiTALK-hosted firmware files, please refer to the URL below:
Version 6.4.3: https://www.obitalk.com/VVX-OE/6.4.3/release.xml
Click “Check for Updates.” And Click on Install
Note: Disregard the version check
OBI EDITION SOFTWARE TO UC SOFTWARE
When you identified that your VVX x50 desktop phone runs the Polycom OBi Edition Software and you need it to have it run UC Software, you can follow these steps below:
Power-up the phone and have it connected to the internet.
Determine the IP Address of your phone by going to Main Menu > Settings > Network
or
Main Menu > Product Info > IP Address
Use the IP address to access the local web page of the phone. (Default username and password is admin and admin, respectively).
Browse to System Management > Auto Provisioning > Auto Firmware Update and set these parameters:



Ensure you download the Split version of the UC Firmware to a local PC running a HTTP or FTP server and the name of the file depends on the model number aka:

VVX150 3111-48810-001.sip.ld
VVX250 3111-48820-001.sip.ld
VVX350 3111-48830-001.sip.ld
VVX450 3111-48840-001.sip.ld
Method = System Start
FirmwareURL = (Enter the URL of a server that hosts the UC Software firmware files)


VVX 150
http://downloads.polycom.com/voice/..._release_sig_split/3111-48810-001.sip.ld

VVX 250
http://downloads.polycom.com/voice/..._release_sig_split/3111-48820-001.sip.ld

VVX 350
http://downloads.polycom.com/voice/..._release_sig_split/3111-48830-001.sip.ld

VVX 450
http://downloads.polycom.com/voice/..._release_sig_split/3111-48840-001.sip.ld

Press “Submit” (at the bottom of the browser window) and then press “Reboot” (from the top right side of the browser window) at the device local webpage at your internet browser.
Allow your phone to reboot and update to UC Software.

Note: If OBi version is 6.3.1 or later the background firmware upgrade is enabled by default. That means the OBi software cannot accept sip.ld firmware file format (only .fw) and so the OBi to UCS conversion cannot work unless you disable the background firmware upgrade. To do so you have to append the string ,bgu-conv=disable.

i.e. FirmwareURL = VVX 150
http://downloads.polycom.com/voice/software/UC_Software_6_4_1_release_sig_split/3111-48810-001.sip.ld,bgu-conv=disable


VVX 250
http://downloads.polycom.com/voice/software/UC_Software_6_4_1_release_sig_split/3111-48820-001.sip.ld,bgu-conv=disable


VVX 350
http://downloads.polycom.com/voice/software/UC_Software_6_4_1_release_sig_split/3111-48830-001.sip.ld,bgu-conv=disable


VVX 450
http://downloads.polycom.com/voice/software/UC_Software_6_4_1_release_sig_split/3111-48840-001.sip.ld,bgu-conv=disable