Hi,
that depends on which kind of WAN connectivity you have, you will need to discuss with your service provider if the option in available.
For example, on ATM links different PVCs can be provided by the ISP, and you can use each of them for a specific type of traffic.
For E1/T1 you can split the channel-groups and so on.
If ISP cannot provide such service, then perhaps you may just add CBWFQ QoS to reserve bandwidth for specific type of traffic. See for example:
http://www.cisco.com/en/US/docs/ios/12_0t/12_0t5/feature/guide/cbwfq.html
Cheers
Marco