It is possible that the call audio is not being routed to the hearing aids. There are a couple of ways to do this:

• When the phone call ringtone is heard, answer the call using the push button of the hearing aids instead of the phone. This will ensure the call is streamed to the hearing aids
• Configure the iPhone to route the call to the hearing aid by default. See configuration options of the iPhone
• During a call, the routing can be switched to the handset by following these steps:

1. From the call screen, select the Audio button
2. A menu will pop-up showing all routing options
3. Select your hearing aids from the list
4. You should now hear the caller through the hearing aids

Other potential ways to address this are: 

• Check reception on the mobile phone and move to a location with better reception if necessary
• During a phone call, your voice is picked up by the microphones of the hearing aids. Check the microphone ports on the hearing aids for debris. If there is debris, use a brush or cloth to clean the microphone ports
• In a very noisy environment, the hearing aids may have trouble picking up the caller’s own voice. To resolve this, relocate to a quieter environment or alternatively route the call to the phone and directly talk into the phone microphone normally.

If this does not work, delete the pairing from your phone, reboot the phone and hearing aids, and then re-pair them to your phone