Trunk channels number

Hi, here is my problem: I have an external SIP trunk, and I'd like to allow 2 simultaneous active calls (in or out), but with the limit of 1 outgoing call.
Example:
2 IN, 0 OUT >yes
1 IN, 1 OUT >yes
0 IN, 1 OUT >yes
1 IN, 0 OUT >yes
0 IN, 2 OUT >NO
2 IN, 1 OUT >no

In general:
(IN+OUT less or equal 2)
AND (OUT less or equal 1);

How can I do? Is there any way to do that through the web interface?

Thanks in advance.

PS sorry for "less or equal" but I can't find the escape sequence