Who is the WAN solution provider and what is the WAN solution? Most true WAN solutions like MPLS honor QOS tags. If you are using plain internet with VPN, that's not really a true WAN service. If you have a company like Time Warner handing you a layer 2 connection or point to point, then your QOS tags are passed but ignored and will come out the other side. This is fine since the traffic entering and leaving each site should be already queued. The tough points for VoIP are at bottlenecks like your router. A 10M point to point or layer to ELAN is just passing the traffic from one end to the other without doing anything to it and there shouldn't be any bottlenecks in between the two ends. Generally in these cases they'll say they don't support QOS for a reason. They don't want the responsibility of troubleshooting QOS issues which are pretty much guaranteed to be on one of your two ends.
Even if you're just using internet with a VPN, you may be alright. It depends on the geographic location. Although they can't and won't support (ie troubleshoot) your QOS on an internet connection since they have no control over the internet, there's still a very good chance it will work because you have the same provider on both ends. There's a very good chance that the traffic never leaves your provider's internet egress point and instead stays on their network the whole way. Since they often support QOS for many other services, there's no sense in complicating things by intentionally stripping your QOS tags. Instead they are likely just ignored. This means that you may occasionally end up with quality issues when faced with bottlenecks such as shared service cable modems at peak periods where all the traffic is hitting the node at once and it doesn't care if your packet is QOS tagged or not, everything is going in and out first come/first serve.
As an enterprise telecom rep, this is a real pet peeve of mine. People often buy these VoIP systems and think "Hey, I can just get all these free calls!" and then run into trouble with call quality issues and blame the carrier, or VoIP in general. VoIP is intended to function on a controlled private network and then connect to the PSTN through a carrier where the traffic isn't co-mingled with internet data. The fact that it works over the internet is nice but comes with drawbacks.
If I were you, I would first just hook it up and see what happens. If you have trouble, call and get an engineer at your WAN provider on the phone and ask them if they strip the QOS or if they ignore it. If you are on a managed VPN or something I would ask about any other WAN or P2P options they may have. Don't talk to a sales or support rep. They will most likely be small business reps with limited products for small businesses. You want enterprise, major accounts, AE2, AE3, etc people who are the larger business people who sell complex products. I guarantee you if you called a Time Warner small business rep and said you wanted a layer 2 100M any to any private network or dark fiber, there's a 75% chance they wouldn't even know what language you were speaking.