I posted the thread below titled "If You Plan To Work In Telecom" that provides links to data networking certification resources. Once you understand how data networking is layered by the OSI model, you need to understand how CenturyLink services use the OSI model to provide connectivity to customers.
CTL is presently offering Ethernet and GPON with IP assignments to customers for OSI layers 1,2, and 3 connectivity to the internet and cloud systems. You will understand how this works with the broad subject certifications. However, if you want to continue working at the customer premise, you will also need to understand the protocols that CTL's services use within the OSI model.
CTL is presently offering SIP service to business customers, and it may be a model for residential products later, after the copper world and central office digital switches are all gone in most areas but the most rural. If we can get a reasonably fast internet connection to a customer now, they won't need costly digital switching in local central offices, and we will offer them very cheap dial tone, voice messaging, and voice/video conference calling on SIP/VoIP connections right at their homes. These services are already being sold to businesses quite successfully, and it's just around the corner for residential customers with broadband connections. Even if CTL falls behind, or chooses not to offer these services to residential customers, our competitors will, and you will be able to change companies to stay employed in residential service if you know what you are doing.
Therefore, if you want to stay employed in residential telecom, after you become certified in data-telecom, the next step will be to learn about VoIP and SIP to understand how signalling and call setup work. The basic principle is that a lot of costly central offices and their digital switches in local areas are being replaced by large PBX type switches in a cloud configuration around the country. This is a cost effective method of delivering dial tone and video conferencing to residential customers that will allow CTL to stay competitive. You will need these skills even if you end up working for any one of the competitive companies who can also establish residential cloud services and offer them to customers anywhere in the world so long as they have a broadband connection.
If you are serious about staying employed in residential and business telecom, you have to make yourself valuable to employers. The more valuable you are, the more secure and better paid you will be. There's no time like the present to make yourself valuable.
in summary, twisting copper is going away. After the OSI model and certifications, lean VoIP and SIP. They actually work hand. Stay employed by becoming valuable to employers.