I work in a call center for one of the largest telecommunications companies in the USA. You already know about it. I'll answer any questions you have and will provide some advice of my own:
1) If you can, make use of a companies/organizations web site, mobile app, and/or IVRs (automated voice systems) instead of interacting with live agents. It can be much faster and more effective. When you interact with a real person you don't know what their level of competence/training is, and you may very well be interacting with an identity thief. I have access to SSNs, DOBs, as well as payment information, and they do as well. The less people interact with your account, the better.
2) Being patient and nice goes a very long way. Most call center work is very stressful and boring. Its unfortunate that some calls can take hours, but that's reality sometimes. Multitask by cleaning or playing a game while the call is in progress. Letting the agent take their time and relax reduces the chance that they'll get something wrong or that they'll fuck with you in retaliation. The systems can be very slow sometimes, have lots of steps, and agents often have to consult internal articles and other information, which takes time to search for.
3) Big companies may have hundreds of departments. The company which I work for does. Some departments can be only reached internally, and only by specific departments at that. You may need to be transferred many times to get what you want. Its unreasonable to expect the average agent to handle billing, fraud, sales, tech support, account support, legal support, and the litany of other subjects that exist. When you DO manage to get to where you need to be, see if you can receive a call-back number, or if the agent can call you themselves.
4) Agents are trained to look out for social engineers, and are required to authenticate callers when accessing accounts or performing specific actions. The internal systems are designed to look out for red flags and may force fraud analyst reviews to proceed. SIM swap scamming is known enough at this point that it has its own response team. Don't be stupid.
5) Differentiating a real employee from a highly skilled scammer can be EXTREMELY difficult. Real call center experience, unauthorized access and abuse of company tools, phishing resources, data leaks, and caller ID spoofing are all techniques that scammers can use to fuck you. When in a call, ask for an agents user/employee ID, an outbound e-mail/text, confirmation of the information on the account, and/or for a supervisor. Doing these will dissuade the majority of scammers.
6) Calls are recorded for quality assurance, and interactions are logged. Document the date and time of contact, the actions performed, and the name/ID of the agent. If anything goes wrong you can resolve it later. Escalation DOES work, and for my specific company, the premium customer support team(s) will have individual agents remain in contact with you for days until a conflict is resolved.
7) The promotions are real. Yes, you will get a free year of Netflix or a $300 card if you (re)order the service. Make sure to document the procedures to receive the promotions, and ask to have a supervisor recap the call to make sure you're actually getting what you've been guaranteed. BE WARY of retail locations and the real life salesmen.
8) Agents talk to dozens, if not hundreds of people in a day. Its unlikely anyone remembers who you were or what you said. Going back to point #1, the environments are paperless and phone-free. Exfiltrating your personal information is difficult but not impossible for agents.