Bell MF tone generation

# What does it do?

The Bell MF tone generation module provides for the generation of the repertoire of 15 dual tones needs for various Bell MF signalling protocols.

# How does it work?

Basic Bell MF tone generation specs:

- Tone on time = KP: 100+-7ms. All other signals: 68+-7ms
- Tone off time (between digits) = 68+-7ms
- Frequency tolerance +- 1.5%
- Signal level -7+-1dBm per frequency