To use a landline or mobile phone effectively with hearing aids you need to ensure the phone is positioned close to the microphone of the hearing aid.  If your phone has a volume control you should adjust this for comfort.

If you wear in the ear style hearing aids you should place the telephone receiver directly over the ear canal as you would normally, if you experience any whistling (feedback) move the phone slightly further away from the ear, this should allow you to follow the conversation without any interference.

In behind the ear style hearing aids the microphone is near the top of the aid so instead of holding the phone receiver to your ear canal you should position the phone closer to the top of the ear.  Some people find it helpful to move the phone about while listening to the dial tone, wherever you hear the dial tone loudest is the best position.

Even with correct positioning some people can still have difficulty hearing on the phone, if this is the case it may be worth trying the loudspeaker option and adjusting the volume for comfort.

Did you know if you have Phonak Marvel hearing aids you can pair your mobile phone for calls?  To check if your phone is compatible and for pairing instruction use the following links

https://marvel-support.phonak.com/en/audeo-m-cell-phone-compatibility/

https://marvel-support.phonak.com/en/android/

https://marvel-support.phonak.com/en/ios/