Voice vlan and access vlan

I have a situation where I have configured switchport to access access vlan 4 and voice vlan 12. Need less to say I have all the necessary vlan configured. I am using Sangoma S500 phones. So When I plug the phone in Its pulling ip from the Data/access vlan.

Phones are capable of doing lldp and I have that enabled. I have tried using being that enabled as well but no different result being seen.

I know this work for Cisco phones but why not any other phones. I have seen similar issue on aastra phones. Due to short time to do research I interchange the vlan value and it worked. I don't want to do this for learning purposes.

Switch is cisco 3750G and below is the switchport configuration.

"switchport access vlan 12
 switchport mode access
 switchport voice vlan 3
 switchport port-security maximum 2
 switchport port-security maximum 1 vlan access
 switchport port-security violation restrict
 switchport port-security
 srr-queue bandwidth share 1 30 35 5
 priority-queue out
 mls qos trust device cisco-phone
 mls qos trust cos
 auto qos trust
 spanning-tree portfast"

Hopefully I will get this clear with your help.

Thanks