Typically phones learn the correct vlan to be in by either a dhcp option or a link discovery message (CDP or LLDP-MED).  If you are not using a link discovery method, then you need to have a dhcp option set that tells the phone what vlan to use.  You need to make sure your switchports are set up as trunks with your data vlan as the native vlan.  The phone boots into the data vlan, gets a dhcp address, with an option to tell it the voice vlan and to tag that vlan.  The phone reboots and tags its traffic and gets a new IP in the voice vlan.

Kind of an ugly set up.

If it uses link discovery, you use a network policy or the voice vlan command to assign the voice vlan and the phone learns the voice vlan via the CDP or LLDP packets.