I have this very weird problem and I just can’t fix it.. any ideas would be very appreciated!

I have a Cisco 7965 ip phone here on my desk. I have a tftp + sip server running in a datacenter and it seems the phone fetches all the data from the tftp, but can’t connect to the SIP server.

Now here’s the weird thing..

If I run the tftp + sip in the same local network with my cisco phone, everything works perfect! I can call outside and receive calls.
As soon as the tftp or sip is outside (datacenter) I’m having this problem.

To make it even more complicated, I’ve tried with iphone app Softphone, and then the external SIP does work!
I’ve checked and double checked everything and it all looks okay.. so any suggestions what to do? (yes I have changed remote IP’s + NAT settings)

Thanks in advance!