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(Nope, if you're a gamer, it's not Samurai showdown 7.) :)

Telcos and mobile services are still booming here in the Philippines with no signs of abating.

Surveys (based on businessworld 2003 yearbook) ranks Globe and Smart as top 10 & 11 respectively, as the country's top companies. (And from what I've heard, they've got great benefits. 15-16 months pay). And with PLDT in 7th place (boasting as the country's best manage company of 2003).

So what makes these companies' systems tick??...
It's "SS7" Signaling System 7 or C7. PSTN (Public Switched Telephone Network) relies heavily on this.

Developed by AT&T in 1975, SS7 or C7 is a protocol suite that is used globally.

In the public switched telephone network (PSTN), Signaling System 7 (SS7) is a system that transports the information required to set up and manage telephone calls by converting signaling information to digital packets. An international telecommunications standard, SS7 uses out-of-band signaling, meaning that signaling (control) information travels on a separate, dedicated 56 Kb/s or 64 Kb/s channel rather than on the same channel as the telephone call. Historically, the signaling for a telephone call has used the same voice circuit that the telephone call traveled on (this is known as in-band signaling).
http://www.intel.com/network/csp/solutions/ss7/7194ovr.htm

And talk about convergence...it can bind fixed-line, cellular, and IP Networks together. So, want to work in a telco? study SS7. :)

Very good read here - Signaling System No. 7: The Role of SS7
OpenSS7 - Open SS7 FAQ.