Install something like gizmo (gizmoproject.com) on the laptop then use
sip:email@example.com or **768(yournumber) to connect back to base.
Gizmo uses sipphone so you will get a sip service you can use anywhere.
The laptop can't use plustalk if it is connecting via a non plus connection (I.E. over seas)
I see sipphone is peered with plustalk so the two should talk easily.
I just tried from Gizmo to plustalk but looping back to another phone on the same network is not a good test (it workled but I couldn't sort the audio)