Yes, Digium phones are hearing-aid compliant, for more information on about the phone specifications please check its datasheet.

What Makes a Phone Hearing Aid Compatible

Hearing aids operate in one of two modes – acoustic coupling or telecoil (inductive) coupling. Hearing aids operating in acoustic coupling mode receive and amplify all sounds surrounding the user; both desired sounds, such as a telephone's audio signal, as well as unwanted ambient noise. Hearing aids operating in telecoil coupling mode avoid unwanted ambient noise by turning off the microphone and receiving only signals from magnetic fields generated by telecoil- compatible telephones. 

A telecoil is a small, tightly-wrapped piece of wire inside the hearing aid that, when activated, picks up the voice signal from the electromagnetic field that leaks from compatible telephones. While the microphone on a hearing aid picks up all sounds, the telecoil will only pick up an electromagnetic signal from the telephone. Thus, users of telecoil-equipped hearing aids are able to communicate effectively over the telephone without feedback and without the amplification of unwanted background noise.

Telecoils can only fit in two styles of hearing aids: "In-The-Ear" and "Behind-The-Ear" aids. Most smaller hearing aids are not large enough to fit the telecoil. Many people report feedback (or squealing) when they place a telephone next to their hearing aid. When placed correctly, telecoils can eliminate this feedback because the hearing aid microphone is turned off and the hearing aid only amplifies the signal coming through the telecoil.

What's more in newer Hearing aids it should switch on automaticly while in older hearing aids you will have to refer to there documentation how ever no modifications are required on the phone which generates the required fields automaticly. Making it so that both a person with a hearing aid and one without can use the same phone without any noticeable difference in there sound quality.