There may be a notch filter at the precise SF frequency, either filtering the circuit at all times or only when the circuit is off-hook, to prevent the user from inadvertently disconnecting a call if the users voice has a sufficiently strong spectral content at the SF frequency, a falsing condition known as talk-off.
Notoriously, this property was exploited by blue boxers and other toll fraudsters.
The SF tone may also be just outside the user voice band, e.g., 3600 Hz.
The conversion into tones, or vice versa, is done by SF signal units.
[1] It declined in the 1970s due to the adoption of T-carrier, and was largely abandoned late in the century in favor of common-channel signaling.