Yes, you are right internet connection affects the VoIP phone quality. But the voice clarity also depends on the locations. The location plays important role in CSR (call success rate). The call setup success rate is key performance indicators used by the network operators to determine the performance of their networks. Choice of codec also affects the VoIP phone quality because you need enough bandwidth to make good quality calls. The VoIP hardware equipment you use greatly impact on your quality. Poor quality equipment is normally the cheapest; therefore try to gather as much information as possible on an ATA, router or IP phone before investing on it and starting to use it. It might also be that the hardware you choose is the best in the world, but still you get problems – the reason being you are not using hardware that suits your needs.
I am also using VoIP services from …. So far, I have not faced any issues like poor sound quality; not even software or hardware problem.