In reply to a post by cjn:

On my N300A, Telephony / Audio shows an option "Allow 1 VoIP call only" --- Yes / No
This should do something useful.

Ah - yes, that shows on my N300A also. The text says "the following settings allow you to adapt your Gigaset to your individual DSL connection." So if you have a slow connection this would maintain performance. But presumably, if you don't want two or more calls at once, turn this on.