Many phones now have a volume control. Some are specially designed for people with a hearing loss. The most powerful of these may amplify the signal as much as 30 decibels.
Some phones also have other special features of interest to people with a hearing loss such as: mini-plug jack (for plugging in a headphone, audio-loop or DAI), frequency control, and extra loud, adjustable ringers. Phones that do not have amplifiers built in can usually be adapted with an add-on telephone amplifier.
A telephone amplifier can usually be added to telephones that don't have one built in. Some simply fit over the ear piece and are powered by a small battery. These can be convenient and portable, but they come in all levels of quality ... some so poor as to be essentially useless. If you need a good portable strap-on amplifier, be sure to buy one of top quality.
Other styles can be inserted "in-line" between the phone and the handset. Some of the adaptors also provide a stronger magnetic signal so that a tele-coil can better pick up the signal. Some also provide not only increased volume, but a way to boost the high frequencies more than lower frequencies.