Pros
May be a valuable initial experiece for someone fresh from school, to learn how a large company operates (with all of its ups and downs). 37.5-hour work week (officially), flexible hours (to some extent), some telco-specific benefits (mobile and ADSL fee discounts) and some very skilled people at the bottom (i.e. technology) level. Redundancy payment (applies only to those who are long enough with the company), although this can be listed among the cons as well, especially for positions that are not strictly bound to the telco technology and for employees who regularly deliver, because they will hardly ever become redundant and without this "benefit", many more would have already left for another job.
Cons
Salary not competitive (with huge differences at the same positions between employees coming from different previously acquired companies, neither a plan nor intention of the management to close this gap), virtually no career opportunities and a very limited professional growth (this wasn't always the case but is getting increasingly worse). Company is no longer able to retain talent and is seriously lacking in both short- and long-term strategy and vision. Perpetual splitting of the company into smaller parts with regular layoffs and without proper communication to the employees results in unnecessary levels of stress and accelerated burnout in those who stayed.