Loop issue with Cisco 3750 and a polycom

So i came from holiday to find a bunch of users were complaining of slow connections and jitter while talking on the phones. upon checking i found they were all connected to the same switch. CPU was 100% , i found ARP Input was eating all of it. upon doing some more digging i found the switch was looped between two ports.

So when i went to check what caused the loop it was a polycom phone that had the PC port connected back to the switch.

Switch f0/1 ----------- polycom -------- switch f0/2

Now what id like to understand is how the switch couldn't block the loop since it had BPDUguard on (it was seeing itself through CDP) and also What protocol would prevent this kind of loop?. My guess is that the phone was not forwarding the BPDUs to the PC port and switch was not aware of the loop.