Magnus,
What you will need to do is tell us the version of software your Mitel 5000 is currently running; there may be a known issue with SIP on your version.
Next, have you looked at your SIP logs to see if there is an issue with the SIP protocol handshake to get the calls in the first place?
Darrel,
That is an unfair assessment of a product since it is the duty of the person setting it up to ensure that the system the softphone attaches to is secure; the softphone itself is not the issue most of the time.
Thanks,
TE