We have no QoS enabled… That’s a problem we would have to address…The question remains if this is a technological limitation where this type of setup is not able to provide this type of service?
From the description in your question, I’m assuming this is an IPsec VPN through the internet that utilizes each site’s primary DSL connection. VOIP communications are possible with this type of connection, but the first thing that comes to mind is QoS.
Your two Vlans are probably Voice and Data, but a Vlan will not guarantee that voice packets get the bandwidth and priority that are required.
I’m making a lot of assumptions about your setup, but I think you should look into setting up QoS.