enum DtmfSenderLowFrequency

Possible DTMF low frequency values.

References from:: DtmfSenderService;

`697`	`FIRST_LOW_FREQUENCY`
`770`	`SECOND_LOW_FREQUENCY`
`852`	`THIRD_LOW_FREQUENCY`
`941`	`FOURTH_LOW_FREQUENCY`

enum DtmfSenderLowFrequency