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.
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The majority of the apps actually ship with their own data generation tools. The PCI and ESS suites are not free to download, you will have to contact sales and arrange a demo, as they are paid add ons with bundled professional services.
Other apps however, such as the Splunk for Cisco Security app are free and include data generation tools.
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