There's so much you can do in Telco.
To name a few, you cna do push marketing, you can create areas, such as O2 Germany, called home areas which allow you to use your mobile inside certain areas as the cheaper home rate, then track the phoen to charge them approiately if they use it out of the area, you can check out the demographics of your customers, locations. You can track a phone user (useful for keeping tabs on kids, workers, company assets, vehicle tracking, etc).
In short, pretty much everything you can do in business, you can do with Telco's. One of the things you should do as a practitioner, expecially freelance, is to expand a clients understanding of what is actually possible. Not only do Telco's have the address information of their clients, they actually know where they are, if their phone is turned on. This opens up endless development of that location data.