Sprint

The quality of your connection (number of calls dropped, the sound quality) changes over the time of your contract with the company. Also If you request them to honor a promise of free phone, rebates, or other savings, that they decided to conveniently forget, then you will notice, very immediate drop in quality of the sound and increase in number of calls dropped. The change is so instantaneous that one almost suspect that there is a routine that computes the amount of money your account is paying, and will be paying, and changes the quality of your call based on that.