My guess would be that it does not support voice calls. (Now here I go speculating 
) But the absence of any indication of voice support from the information I can see, combined with the fact that they recommend it for everything except phones, would be consistent with that.
edit: Although it does say “Voice: Optional” for this modem on page 14 of the brochure, so maybe 
Oh I’m interested in tons of things, including the phone, the laptops and computers just keep me plenty busy as it is, others have much more background in the L5 modem stack 