QSIG on the TDM leg between the routers will also help facilitate some of the enhanced features PBX to PBX. Although Q.931 is a great signaling protocol you may need an external DS0 to link the two PBXs as Bill is alluding to. Many PBX vendors have a proprietary way of communicating between PBXs. QSIG is a way to bridge that gap. Included in the QSIG spec is how to signal Message waiting indication(MWI).
The EYM circuit that you are talking about, I am assuming is actually E&M. That being said E&M does not have enough signalling states to emulate advanced features such as MWI at least between PBXs.