I guess you can say they have peer roles, but they are just different. They control different networks. At the end of the day protocols are just protocols, It is simply the language the devices use to communicate.
You have your native language as do I, in order for us to work efficiently together we must speak a common tongue(In this case english). They probably wouldnt sit all in a single network with protocol translation going on somewhere, but the are more like todays global communication.
In France, folks speak french, just as in spain they speak spanish and in the UK they speak english. How do folks understand each other though when they want to communicate? They speak a common tongue, maybe they all speak french or maybe they all speak english or spanish, it doesnt matter. All that matters is that they understand each other in order to get the data across. This is the same with VoIP networks.
I could have a SIP network, h323, ss7 as well as MGCP network, but none of them can work together until they speak a common tongue. You will see that most VoIP gateways speak the common tongue of TDM on one side. It could be E1 PRI, T1 PRI, E1-R2 (variant) or T1-CAS, whatever it doesnt matter as long as the attached switch understands it. Same with VoIP, a 323 network cannot speak to a SIP network natively, they must use some sort of protocol translator or the endpoint must understand both protocols.
MGCP is a different story, since one element (call agent) is “controlling the whole network, it must understand either SIP or 323 in order to talk with those networks. Call Agents cannot speak MGCP to each other because they are peers and MGCP is designed as a master slave type relationship.
Hopefully that clears it up, if you have any further questions feel free to email firstname.lastname@example.org