In the past few months, participants of several math conferences and schools have been approached (often by phone) by some "agency" asserting to be involved with the hotel reservation and requesting credit card information and the signing of some electronic document.
No such agency has been commissioned by us. We will contact you via email (from addresses @algebraic-geometry.de, @mathematik.tu-darmstadt.de or @uni-due.de). We will not ask for credit card numbers or other payment details.
Please do not hesitate to check back with us if there is anything you are unsure about.