2010 ADA Standards for Accessible Design Pocket Guide
The International Symbol of Accessibility shall comply with Figure 703.7.2.1.
The International Symbol of TTY shall comply with Figure 703.7.2.2.
Telephones with a volume control shall be identified by a pictogram of a telephone handset with radiating sound waves on a square field such as shown in Figure 703.7.2.3. [S.H. 4.30.7(2)]
Assistive listening systems shall be identified by the International Symbol of Access for Hearing Loss complying with Figure 703.7.2.4.