Hello APS,
As I mentioned in a previous post this is a NAT related issue. I have provided a link to a post documenting similar experiences with a different provider:
http://forums.whirlpool.net.au/index...oIP_engin_5m32
There is no magic solution here, as each case is usually subtly different in some way. My suggestion is to try each of the options posted in the link above until you find one that works for you.