Incoming calls / Extension question

[Spyklee]

Hello,

I've the following configuration
- Patton 4554 with two ISDN lines
- 3CX server (Small Business Edition)
- Several LinkSys SPA942

I've configured a LinkSys SPA942 as extension #102
and fully configured Ext 1 on the LinkSys Ext 2 -- Ext 4 are only set to line enabled: yes

Let's say make an outbound phone call ... any new incoming calls are not shown on the LinkSys.
What are the specific settings I need to set, so I'm able to receive an incoming call although I'm having single/or multiple active calls (on hold).

Cheers! Eric