Switched telephone network (PSTN) services
Fax

On the telephone network, initially dedicated to voice transmission, the fax protocol provides for the possibility of alternating fax transmission and telephone retention within the same call. Today, most telephones/fax machines offer this possibility. While switching from voice mode to fax mode seems almost natural (it's what you do implicitly when you press the fax button after lifting the handset and dialing), switching from fax mode to voice mode can only be done at specific points in the transmission: either at the end of a page, or at the end of the entire document. This function enables both parties to comment on a document they have just exchanged.