As derrick pointed out in the
Halifax Customer Services thread, if you visit the Halifax website, you can ask them to call you back. However, this might not always be convenient.
On the Halifax website it has contact numbers for each area, then it has a section called "International dialling":
For all product areas - 0044 1133 809 574: 24 Hours
For advice on how to use the Online Service - 0044 1133 851 888: 24 Hours
That might be worth a try - but be warned, "calls are charged at international rates"!