The current Australian Standard for telephones limits the gain to 22-24dB for the handset earpiece. Any Standards compliant (read, legal) telephone that is advertised with greater gain is referring to the output from the speaker, not the earpiece.
What you can do though, is add a Portable or In-Line Amplifier so that the sound of the phone can be made louder.
An amplifier will also increase the magnetic field strength from the earpiece so that you can use the telecoil function of your hearing aid or implant when this option may not have been successful before.
If you use a hearing aid on the microphone setting, the hearing aid will 'compress' the amplified, louder sound from the earpiece and undo what you are trying to achieve.
If you do not have a telecoil function in your aid, then you need to remove the aid and just use the in-line telephone amplifier alone.