VOIP + MyISDN limit incomin call when 1 extension call outcoming

Hi all,

i need some suggestion…my current configuration is:

1 trunks sip euteliavoip (only for outgoing calls with hidden caller ID)
1 trunks myISDN IN and OUT ( 2 numbers available on my ISDN device)

5 Extension (IP301 )

Everything works fine, but i’ve a problem:
when i’m at office and i made an outgoing call it’s ok, but if someone calls me on my ISDN number at the same time, he got a free tone but i cannot answer coz i’m busy with the first call.

Even if only one extension is busy (VOIP or ISDN) i need that for every incoming call the system replies with a busy tone.

Is there someone who can give me a suggestion about that problem?

thx a lot