Having a volume control allows the user adjust the volume of the audio output. This is a useful feature for people who are hard of hearing. Note: many products with volume control do not provide any amplification, compared with the gain of a standard telephone.
Having a volume control addresses:
1193.43(e) Availability of auditory information for people who are hard of hearing.
Product Types that generically include a volume control:
Analog and digital wireless telephones.