In North America, two 25 MHz bands are allocated to Advanced Mobile Phone Service (AMPS), one for "forward" (base to mobile, 869-894 MHz) and one for "reverse" (mobile to base, 824-849 MHz). Each of these bands is split into two, allowing two operators. An operator is allocated 12.5 MHz in each direction. Each channel is allocated 30 kHz. If 21 channels are allocated for control, what is the total number of voice calls per operator?