Most of the hackers I know gave up on Phreaking once hacking became popular in their circles. To me, there will always be something more fascinating about the telephone infrastructure.
It feels magical to know that the same transport that delivers the sound of your voice was susceptible to tampering and rerouting by other sounds.
This is something other than phreaking, imo. It's straight forward service fraud and hacking. Bearer and Signal have been divorced. The attacker has to get privileged access to a network that is generally not accessible to the public. Phreaking was neat because literally anyone could do it if they knew about the methods.
Knowing the methods used here isn't enough. You need to get on the trusted SS7 network and have a roaming/interconnect agreement to start doing really interesting things.
Any sites you'd suggest?
https://www.hackcanada.com/homegrown/telecom/index.html
https://www.hellboundhackers.org/forum/42-phreaking_0.html
http://null-byte.wonderhowto.com/how-to/phreak-basics-016101...
http://web.textfiles.com/phreak/phreaking.txt
http://www.aboutphone.info/lib/phreak/boxes-2.html
http://www.textfiles.com/phreak/BOXES/
That should give you a good head start. . .
I read all this as a kid... :)
[0] http://explodingthephone.com/ (AudioBook also available)