Media traffic volumes are generally not visible, because they're from endpoint to endpoint, so unless you've got really detailed monitoring (which the original poster said they didn't), you're not going to see traffic between two phones in the same building, or traffic between buildings that don't have the call manager in them. Obviously the measurement problems are different for ISPs, enterprises with IP-PBXs, and VOIP companies. Also, the amount of media volume not only depends on the codec, but also on the length of the call, while the signalling volume mainly depends on the number of calls. So if your customers are averaging 3 minute calls, that's a much different ratio than if they're doing 10-second credit card validation calls or one-minute voicemail pickups or 60-minute teleconferences. If you had enough measurement capability to estimate this, you could use that directly instead of guessing from signalling traffic, but otherwise you're just guessing.