By Jess Jones Telecom giants Vodafone and BT are set to report full year results this week amid a battle to arrest a slide in their share prices over the past year. Investors will be keen to get an update on Vodafone, one year on since its new chief executiveMargherita Della Valle declared that “Vodafone must change,” saying its performance had not been good enough. The telco veteran has wasted no time carrying out her restructuring, which involved selling off its weaker Italian and Spanish divisions. A proposed merger of its UK business with Three UK is awaiting regulatory approval. Excluding…