Native speakers can speak quite quickly, which makes it difficult to understand. This is a simple phrase to ask the person to slow down.
(Note: This phrase is often used in spoken language, but it is not grammatically correct. The correct wording should be “Could you please talk more slowly?”)
A: You can give us a call any weekday from 8:00 a.m. to 4:00 p.m. at five five six, one five zero seven, extension three three
B: I’m sorry, could you please talk slower?