Many people who currently work in the field of VoIP and IP Telephony, or want to break into this line of work, learn it by building a small lab with cheap equipment so that they can see how things really work, and practice configuring the gear. This is particularly true of those individuals pursuing vendor certifications like the CCIE, which have a lab component.
One potential snag is the various connections between devices, which is of course, what the networking industry is all about. The issue is that sometimes devices can be connected directly together via a special cable, and sometimes they can't, depending on the technology. For instance, it is possible to connect the Ethernet or serial interfaces on two routers together via a "crossover" or "back-to-back" cable in a lab, where in the real world, you would typically have an Ethernet hub, or the entire network of a service provider, respectively. Obviously, a small cable is much cheaper than either a hub or leasing a T1 connection (although it would be entertaining to explain to your telco why you're terminating both ends of a T1 to the same room.)
In a voice lab, one of the most common things you would want to do is connect two PBXs or gateways together. For instance, you might want to simulate connecting a customer's PBX to a router, which is configured as a gateway to your IP network. To do this cheaply, you need to pick a circuit type that supports "back-to-back" connectivity.
A good choice for this is the E&M trunk. E&M is an analog trunk type that is relatively cheap and easy to find. (Go here for more information about E&M.)
E&M is also supported by almost everyone. But it's important to understand that there are 5 types of E&M circuits, and the first one, known as "Type I" doesn't support "back-to-back" connections. If available, choose either Type V or Type II. These circuit types support "back-to-back" connections because they isolate the electrical ground. Other circuit types have a problem because they either have two batteries and no grounds, or two grounds and no batteries.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.