Nokia SIP Client problem

[nobrand]

Hi everybody! I'm using nokia sip client for my voip calls, and i've installed the SIP VoIP Settings plug-in to configure VOiP settings.
I can make and receive VoIp calls through my asterisk PBX, but I have a problem: if there is an active internet call (data Call) , when i try to call the sim number (voice call), i receive the busy tone; on the contrary, if there is an active voice calls on my cell and it receives a data call, this last call is put on hold.
How can I have the same behavior when there is an active data call? I have to change any setting on my cell (E55) ? I thought it might be a bug of the Nokia SiP Voip Setting.
Thanks!!

[traud]

Is Wi-Fi (Wireless LAN) involved at all in your issue?