Cisco Switches :: ESW-500 / SPA525G CDP - Operate Differently Between Phones?

I have an ESW-500, a SPA525G (also tried 504G), Polycom IP330/670 phones, and a Switchvox (Asterisk-based IP-PBX) and CDP causes some issues.Both the SPA5xx and Polycom phones support CDP but it appears to operate differently between the phones. And, surprisingly, worst of all on the SPA's.
 
In short:

* Switch - CDP enabled (VLAN 100), DHCP disabled (Option 82 enabled) which is basically how it comes out of the box

* Polycom phones - CDP enabled (default)

* Cisco phones - CDP enabled (default)

* DHCP/TFTP server on port 1

* Phones on ports 7,15,19 - default as smartports for Desktop & Phone

* Switchvox on port 17 - port configured as General (not Trunk) with 1U & 100U VLANs. This was required to get Polycom phones to appear in Switchvox config
 
With that configuration the Polycom phones work perfectly. They appear in the switch web-GUI under CDP, they present in Switchvox and can be configured.But with that configuration although the Cisco phones appear in the CDP screen they do not get DHCP. My DHCP server can see 'DHCP Discover' messages but it just goes around in circles. Disabling CDP on the switch will allow the phone to get an IP but disabling CDP on the phone still has the same problem. When I saw "around in circles" this is what I mean....
 
Rcvd DHCP Discover Msg for IP 0.0.0.0, Mac 00:25:84:06:12:46 [10/08 13:51:27.031]
DHCP: proposed address 192.168.100.16 [10/08 13:51:29.000]
4064 Request 2 not processed [10/08 13:51:29.062]
Rcvd DHCP Discover Msg for IP 0.0.0.0, Mac 00:25:84:06:12:46 [10/08 13:51:30.093]
DHCP: proposed address 192.168.100.16 [10/08 13:51:31.593]
4064 Request 2 not processed [10/08 13:51:31.656]

[code]....
 
With the difference being there's a 'DHCP Inform Msg' which there never is from the Cisco.I have tried many options regarding DHCP Relay, DHCP Server addresses, DHCP VLAN Interfaces, smartport settings etc but with CDP enabled the Cisco phone just will not get DHCP.