Modern Bluetooth Hearing aids are great but do not integrate with office handsets.  I modified a typical handset and added a 3.5mm output to the handset.  The handset will still function as normal but can transmit to bluetooth hearing aids through a 3.5mm transmitter.  The photos below show the handset with a Widex bluetooth transmitter allowing the phone call to be heard in both ears of a hearing impaired user.  The close proximity of the transmitter to the behind the ear hearing aid allows for perfect call quality and you get the additional boost of normal sound through the handset speaker.

Hearing Aid Handset on EarHearing Aid Handset on Phone DockPhone after 3.5 mm Port Installed3.5 mm port soldered to existing speaker wireWiring to add the 3.5 mm plug