Unfortunately, the answer is a lot of "it depends".
Splunk is obviously great at getting data from all sorts of traditional IT systems via either a direct network connection, or a forwarder looking at a log file.
Some telco equipment is easy, if it is already based on an existing 'server' architecture. For example, Avaya used to make a Call Management System that was mostly just software on a solaris box. That's easy, load a forwarder and go.
But, a lot of stuff is going to be a lot more difficult. Some of these devices have limited, if any, TCP/IP support. Many of them are old enough to simply have a "Craft" RS-232 port where they would hopefully log alarms.
One thing you might consider is using a small terminal-server style device to bridge the gap. I think you can use something like a Digi Portserver - it can be configured to connect to a TCP host/port and basically dump everything coming in over the serial port. And they aren't too terribly expensive - you could buy a small one (4 port) to test with for a couple hundred bucks I think.