Caller display

[#1]

Hi,
I would like to include in my package a caller display.
When I included the feature at £0.99/month, was added £0.54 for cost for the remainder of month. I'm at lost what this cost is for and if will be charged every month.
Could someone help me with this, because Plusnet is not very clear on their pages.
Thanks

[#2]

That will be the pro rata amount for the rest of your billing month, as caller display should be activated next working day.

[#3]

Thanks for you help!