It mainly completes the point-to-point voice communication between neighboring stations, without dialing, just off-hook. This PCM service is called:

It mainly completes the point-to-point voice communication between neighboring stations, without dialing, just off-hook. This PCM service is called:
A . Sub-rate services
B . Z interface extension business
C . FXS hotline business
D . E&M relay service

Answer: C