Call the Gigaset help-desk - they are extremely helpful and know about most issues. They are not UK based but work 09:00 through to 20:00 and are on a UK number 020 3695 3111. I would guess it is related to a firewall of Virgin not routing traffic to specific locations.
I had a problem recently and wit me they worked through the whole set up.
Do you have all of your user details - ID, passwords, various server addresses &c? You can also get a short term temporary "trial" number and settings from many of the VOIP service providers - get a couple of those to try out too.
M H C
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