I am not able to answer WhatsApp / Messenger / other internet calls with the hearing aid push button or double tap.
Some apps, which have the internet call option (for example Messenger, WhatsApp, etc.), do not configure the sound of incoming calls as a ringtone. The sound is not recognized as a call but as standard media streaming, and therefore:
Double tap will pause the streaming and not pick up the call.
Pressing the button on the hearing aid will not pick up the call.
The incoming call can be picked up only on the phone. After that, the call will be routed to the hearing aids. This app functionality cannot be configured on the hearing aids.