Programmers working in telecom operators - what exactly do you do?

Question

Work organization

I mean Rostelecom, MTS, Domra and other companies - i.e. those that provide services on the Internet, television, telephony, etc.
Is the development department in-house or outsourced?
What is your staff of developers, what specializations?
What tasks do you solve, if not a secret?
I'm interested because I myself work in one of the operators in my city with 20k+ subscribers.