Just for my reference and everyone else's:

On the 2820v, intercom phone calls (between the two lines/FXS ports) are dialled as 10# or 20#. (On older Drayteks, it was 01# or 02#).

Release notes suggest that the intercom is now disabled by default. The intercom function is enabled via telnet with

>voip dialplan local 1

and obviously disabled by changing the 1 to a 0.

It would have been nice if Draytek had documented this in the user manual or in the release notes!