To make a long story short: about 10 days before the Orthodox Easter, I started noticing a hissing sound / white noise during calls when using my Lumia 720; people could hardly hear me and I had issues hearing them, too. I tried to be patient and suspecting that it might be due to humidity inside the phone (which would be strange, but still a possible cause of the issue) so I used a hair dryer to make sure that it was dry inside (the 720 does not have a detachable back/cover) but it made no difference at all.
Right after Easter break (on Tuesday 14/4/2015, to be precise) I decided to send it for a check to the authorized service center (Nokia Care @ Alexandras Av.) in Athens. The phone was still covered by a warranty (purchased back in February 2014) and I was informed that I wouldn’t have to pay for the inspection nor for the repair in case it was a hardware issue (and not misuse; e.g. due to a fall or moisture); at the same time I was also informed that most of such cases were assigned to “moisture issues”, a fact that invalidated the warranty and resulted to a cost for the repair. Since I had paid 170 euros for my Lumia 720 and it was already more than 1 year old, I was not willing to pay for its repair; instead I could keep it as a camera/internet “box” and get myself another middle-range Lumia. They told me that it would take them a couple of days to check it and see if they could repair in in-store; otherwise, it would have to be serviced by the central Nokia Service in Athens (Arvato). They told me to call in the next couple of days to see how it goes.
Indeed, after a couple of days, I was informed that my phone could not be repaired in the store and had to go to the central service – there seemed to be a hardware issue. That process would take about two weeks (minimum) during which I would not have a decent phone (as my only backup phones were an old HTC Touch HD (Blackstone) and a malfunctioning LG Optimus 7) but it was my only choice.
I really hard a hard time during these two weeks (see next post) but in the end I received my phone repaired after exactly two weeks – the issue was its microphone. The best thing? The repair was indeed covered by the warranty (no excuses about moisture) so I did not pay a single cent!