Mobile/cell phone quality sucks. It always will, due to physical reality and engineering constraints. Pretty much the entire user population of cell phone users has had to lower their expectations for quality - some of them in the mistaken belief that at some point in the future quality will improve to land line quality.
Phone companies strive to provide "five nines" (99.999%) availability for land lines. Some of that may even be mandated by legislation, in exchange for easement rights to lay their lines across public and private property. I have no idea what availability they strive for with mobile service - the engineering constraints are not promising.
Internet service providers do not normally promise any sort of availability - and even when you can purchase an SLA (Service Level Agreement) from one, it only applies to that small bit of the Internet that they control. VoIP is therefore generally great as a cheap backup to regular POTS, but even at this late date I would advise against it as a primary voice communications.
(For what it is worth, I worked at a VoIP firm and designed the OAM&P system interfaces for a VoIP Class 5-like switch product. I learned a little bit about VoIP in the process.)
These days I write software that tests the implementation of TCP/IP stacks. Recently we demoed our product to a large company whose TCP/IP stack runs on cell phones. They were interested in supporting IPv6 and we have a whole series of tests for IPv6, in addition to IPv4 tests we have. Their IPv6 wasn't yet ready enough to be tested, but during our sales demo they asked to see how easy our product was to use - so we showed how quick and easy it was to set up and run IPv4 tests on their existing stack. Only a few seconds into the testing our product had crashed their production stack. A stack already embedded in presumably hundreds of thousands of cell phones in the field. (We eventually made the sale - but ironically they dragged their heels a bit because they said a cell phone isn't a critical device and they expect users to "reboot" when such situations arise. Consider that inherent philosophy when deciding whether to use mobile phones exclusively.)
My wife and I used to each have a cell phone but we rarely used either one. So we dumped one and the remaining one is off most of the time but is carried for emergencies.
