voice playback through modem speaker jack- FaxTalk Voice/Fax softwareThe Modem Speaker option is disabled based on an internal database setting FaxTalk has for the modem. This is generally done for modems where there is no specific speaker jack on the hardware (such as the MT5634ZBA) as FAXTalk has found in the past that many customers complained about not being able to make out the playback of messages when the modem speaker was used (especially when mounted on an internal modem).
There isn't a general purpose way to "re-enable" the modem speaker option but there is a way FaxTalk can provide this on a per user basis. The customers can contact FaxTalk directly at email@example.com. They can provide the necessary change to override the setting that disables the selection.