Jandyt:Just wait til this year's new Beocom (speaker-'phone) comes out. It is supposed to be compatible.
Ah! Something I was not even thinking about! I would love a speakerphone!
The truth is that I have been considering doing away with landlines entirely... though I have been considering making the jump to BeoLine + BeoCom2 at the same time! Oh decisions!
For now, I think I will simply extract the least used handset, and wait to see what the speakerphone BeoCom looks like.
But, on the other hand... (you get the idea!) 
++++EDIT++++
BeoLine + (5) of my existing 6000 handsets + (1) BeoCom2 + (1) Serene Dock + (1) empty slot for the new BeoCom! 
WHOA! Will my Mk1 6000's work with a BeoLine? <researching>