Ajai,
Check out ss7.com for ss7 details and call processing.. I feel ETSI is the best reference.
About career, well first of all i assume that you are with an operator and want to switch to telcom s/w. If you switch to s/w mostlikely you will be doing testing than development.1. testing will give you more idea on how a product/sw release gets tested but at the same time over a period it becomes mundane job. 2) I also assume that u are in india well telecom testing jobs in UK and US have comedown coz of out sourcing. You can see motorola, nortel, lucent outsourcing testing to Wipro,Infy,TCS and Sasken.
Be clear why you want to move to telcom s/w !!