If you find that your hearing aid is picking up interference on calls, you can use a neck loop. This will keep your phone away from the hearing aid and reduce interference. Or, you can use the loudspeaker setting on your phone.
Some phones – e.g. hearing aid compatible phones and all BT payphones – contain an inductive coupler. When using a phone like this, follow the steps above as you would with any phone. But if your hearing aid does not have a ‘T’ setting, you’ll need to check your hearing aid manual for instructions on pairing your hearing aid with an inductive coupler.