I was trying to disable VoIP on Vigor router using telnet command: "voip sip misc -D 1".
After rebooting all connections on WiFi dissappeared.

Suggestion
Where you add 1 try again same process but add 0
Usally 0 means enable ... so what ever happened should enable it back
Also be careful because i think (not sure of course) each model has its own different commands

On Vigor 2830Vn Plus DB routers telnet command: "voip sip misc -D 1" works perfect.
When You change from 1 to 0 in that command VoIP WiFi comes back to normal, but internal VoIP is enabled.
The problem is that I'm not using internal VoIP on 2850Vn and I want to disable it.
Most of telnet commands on all Vigor routers are simililar or even the same.

maybe this page help you
http://www.draytek.com/user/SupportDLUtility.php#
scroll down end where is the Telnet Command Guide
2009/02/03

babis3g wrote: Suggestion
Where you add 1 try again same process but add 0
Usally 0 means enable ... so what ever happened should enable it back
Also be careful because i think (not sure of course) each model has its own different commands

Usually 1 means enable and 0 disable on Draytek routers.