So this "Press and hold the '1' key" action looks like a feature exclusively belonging to modern cell phones/smartphones. Maybe some special electronic signal/frequency is sent when we press and hold a key on the cell phone keypad.
Plain old corded telephones may simply do not have this feature at all, as verified by the fact that if I subscribe to a telecom's traditional land-line home phone service with a hosted voicemail feature, then I would normally dial some digital sequence, like *98, to access the hosted voicemail. I never see any "Press and hold a key from your corded telephone to access your hosted voicemail" instruction from any telecom carrier.
Just wondering whether Obihai engineers can hard-wire some instructions into the firmware so that when some special digital sequence is dialed from an OBI200-connected corded telephone, OBI200 will translate it into a "Press and hold a key" equivalent, and send it out through OBIBT.