Note: this post might be considered totally useless by some as there is no clear conclusion or solution. It is a work in progress.
I started using InPhonex, a VoIP provider for individual customers, about 2 years ago because I needed a professional phone number in Lima, Perú, and there were very little providers for this kind of number at the time. During the first 6 months, I had disconnect problems almost every week. While discussing with InPhonex support, it appeared they changed their codec, then something was supposedly wrong with my internet line (I had another IP line from Belgium working just fine on the same phone, same internet connexion).