We have also taken the VOIP phone service which, so far, seems to work fine. The only issue that I have found so far is that the time passed down the line is 1 hour behind BST, so GMT etc.I have tried to raise this with the support department who clearly don't have any knowledge of how phones work.
More likely that the time passed down the line is UTC, which is the same number as GMT but is semantically different. You should be able to configure the displayed time and Daylight Savings adjustments on the phone itself. If it is UTC, I would say Gigaclear are doing this right.
Similar with BIOS clocks on PCs. In the Unix and Linux world, the BIOS clock is set to UTC and the operating system calculates the display time, but crucially leaves the BIOS clock set to UTC Many phones have a linux variant as operating system and operate in similar vein, perhaps without BIOS clock, but taking UTC from the line or an NTP server. Of course, this is NIXed by Microsoft who don't leave the BIOS clock alone and work out the display time from UTC in the BIOS [at least not without a registry hack]. This means that on a dual boot machine, Windows messes up the time on other operating systems.
That was my mild rant. But the answer, I think, is to set the time zone on the phone to one with the daylight saving adjustments built in.