Switch configured with voice and data VLAN on one port fails to register the Phone in the voice VLAN, instead it registers the phone in the data VLAN when LLDP is enables on the ports.
VoIP service affected as the phone is working in the data VLANinstead of the voice VLAN, we checked this in the MAC address table and with the "display vlan x" command.
Get a packet capture to demonstrate that the packets from the phone arive with the data VLAN tag instead of the voice VLAN tag, you should see this in the 802.1q ID in teh packet capture.
When Polycom devices are configured to use LLDP they dynamically assign internally a VLAN before sending it to the switch.
Check the brand of the VoIP phones and verify always if LLDP is configured on the ports or the IP phone, if so, disable it to check the bahavior or get a packet capture. The final solution depends on the brand of the phones and how they manage the LLDP protocol.