I believe this will be found in your network somewhere so lets start by chasing that.
A couple of questiions:
1- what is the IP address subnet and gateway of your 3300?
2- What is the IP address, subnet and gateway of your SIP phone?
3- Is there a firewall in between them?
4- If your SIP phone gets an IP address from a different subnet, take a Mitel phone, put it in teleworker mode by holding the 7 key as you boot and putting in the IP address of your 3300, then test with that.
You can also get your dealer to send you Teleworker TNA testing software where you may be better able to troubleshoot the issue.
I'd be absolutely suprised if the issue was not isolated to the network.
Ralph