For a couple of reasons I think there are likely to be a lot of areas where there is overlap. I would draw the line at "is this exclusively a telephony question?"
Areas I think that should be definitely on-topic are:
Questions of network convergence (I want to trunk TDMoE over my ethernet network. Are there significant problems with using CoS and QoS controls to ensure sufficient performance? Do I need to think about anything else?)
Questions about network management of VOIP, QoS, etc. These are network management issues and are closely related to #1 above.
Questions about WAN telephony carriers, used to carry data and voice together (everything from T1 frames, perhaps ISDN PRI's etc) to the extent the question is about allocating or managing joint resources.
Areas I think should be off-topic are:
Telephony management questions. "I put 4 T1 cards in my asterisk server and I can't get good call quality. What am I doing wrong?" The answer is "replace with a quad T1 card to drop the number of IRQ's" but that really doesn't have much to do with network engineering per se.
Telephony network and interface layout questions, including but not limited to dial plans, IVR's, etc.
Between these there is a huge gray area. Consider questions of H.323 gatekeeper management. Is this a telephony question or a network perimeter control issue? My own preference is to treat gray areas as on-topic. This allows someone to ask a question in whichever community he or she thinks best represents the specific problem that is occurring (and may indeed ask different aspects of the question in both).