Meraki switches do not support "auto voice vlan" so essentially you need to make a custom option in the default VLAN's DHCP scope for Option 125, this will tell the phones to "jump" VLANs to the voice VLAN, or to make it easier, use option 43 using the same string as option 125 (but it doesn't need to be an encapsulated option, it can be plain text).
This is typically why we don't use Meraki switches in any voice deployment network, no CDP/LLDP auto VLANing. Cisco small business series (for example) handles this 10 times better than a Meraki MS switch.