Numbers beginning 055 are Corporate Numbers. Those starting 056 are for Location Independent Electronic Communications Services (VoIP).
The Santander ones, which begin 055114, are charged the same as 01, 02 and 03 calls by BT, meaning that they are included in packages. TalkTalk might do the same, although I only checked the
Talk Anytime UK plan, which does indeed regard these numbers as "UK landlines" for billing purposes.
Which provider are you with and which package are you on?
There are lots of Santander branch numbers in our database, but only one is verified. Please can you confirm that the number is for the branch in question (assuming you do dial it) and report back so I can put it in the listings as verified? If the 05511 number is in the 01/02/03 column (as unverified) I will move it out into the Other Information one as these numbers aren't treated as 01/02/03 numbers by all providers.