Training for GIS professional working in Telecom?

It sounds like you're asking "what GIS subject matter theory should I learn?" and "how can I go about applying it?"

The needs of a Telecom will appeal to various niche-applications of GIS. So you should approach learning the concepts and jargon of the relevant niches (i.e. gaining Subject Matter Expertise), then once you get the concepts (many of which you will probably have a working familiarity with), you segue into applying the GIS by incorporating your more generic GIS skillset.

In my experience, most sophisticated applications of GIS involve several people---call it interdisciplinary---where the GIS Specialist/Analyst is tasked with identifying the goals of the GIS and investigating some approaches to a successful implementation. This can mean gathering together experts and picking their brains---asking the right questions and gathering feedback. Alternatively, you can pursue a court of self-study into the various niches.

The following niche-applications of GIS are particularly relevant to Telecom:

• AM/FM (Automated Mapping/Facilities Management 1, 2, 3)
• Location Based Services (Applications of this could be helpful for field techs, or in reverse, if you're doing fleet-tracking.)
• Marketing (Even if you don't use ESRI, many of these concepts are appropriate, for example the chapter demonstrating a GIS analysis to support a direct-mail campaign.)
• Cartography (If you're making maps, even dynamic ones, make an effort to burn in some degree of sophistication. And don't forget that even good maps tell white lies.)

And for your GIS-specific, more generic skills, basically, Vector data management is going to be huge, as well as most any familiarity with SQL and RDBMS techniques---as the most appropriate solution will put your geodata in a server/database, then merge/join it with the attribute data your circuit engineers are sure to have on hand.

While accomplishing the entire educational task may be broad in scope, it's definitely not impossible.