Hi - I am trying to help my parents out here but am a bit stumped as to where this issue could be. They have recently switched to Vodafone Fibre X. All is working very well with no problems.

They also subscribed to Call Waiting.

Call waiting is "sort of" working - 

1. They are on a call so the landline is in use.
2. Another call is incoming and they hear the beeps to let them know a call is incoming.
3. They press the R button on the cordless phone and they are transfered to incoming call. The original call they were on stays on hold.

Now here is issue1 - They cannot transfer back to the original call. Pressing the R button again to go back does nothing so the original call continues to remain on hold and they have no way of returning to that call.

Issue2 - The caller thats been placed on hold doesn't hear any indication that they are on hold. Just completely blank. Is this normal?

They are using these Panasonic cordless phones - https://www.panasonic.com/nz/consumer/telephones-home-monitoring-video-intercoms/telephones/twin-handsets/kx-tgh222nz.html. Recall/Flash time is set to 600msec as recommend in the manual for NZ.

Is this a problem with Vodafone's call waiting service? 

Any pointers on what can be done to resolve this?

Thanks