First, do you need VoIP or can you use T-1 interfaces directly between the two switches? The reason I ask is that I have prepared several proposals and engineered a few schemes comparing the two. Right now, VoIP seems more expensive even when you factor in a multi-site situation (i.e. more than 3 sites).
If you don’t have to have VoIP – do compare the pricing. You can use copper or fiber for either solution. I know this is a forum that pushes VoIP but when it isn’t economically feasable, it just doesn’t make sense.
[Webmaster’s note: It’s not really true to say that this site pushes VoIP. In fact, our white paper The Drivers for Voice over IP takes quite a skeptical view. So, I agree with the cautious approach that you suggest :-)]
If you have to use VoIP for some reason, then I would first check with the mfg of the phone switch equipment (PBX, Key System, or Hybrid system) and see what VoIP interfaces they support with your current telephony hardware. You may find that you only need an interface card or two per switch that will give you an Ethernet interface with 4 or more virtual VoIP circuits. The next step would be how to interconnect the two Ethernet intefaces. If you hop them onto any existing LAN’s, then you will need to configure your LAN so that bandwidth for the two switches are guaranteed somehow. You can use VLAN, Packet Priority, or IP Subnetting techniques to do this. I would probably just vote for the KISS method and use a private Ethernet 10/100/1000 switch on both ends with Gig Ether fiber SX between the two on. If that is too cost prohibitive for just the two switches then I would look at VLANing them and using the Etherswitches for my other Data also on another VLAN.
Hope this gives you some ideas…