I have spent years in VOIP. Wired lines are Ok, you can get best results. But on G3/G4 lines conversations are sometimes tricky to mantain. Personally I have lost 2 years I a pjt in which the customers obliged me to switch to VOIP. The tests were made in an area near Rome where signal was weak and crowded, while in Modena signal is strong and stable. Be careful: I have lost € 30.000 of work. You can build the best App, but it's nothing if environment is bad!