Complex sentence: After an initial period distributors cease calling to people who do not purchase, and only call at selected customers, (customer base) so the initial stage of blanket dropping is used as a filtering process to find customers.
Split into simpler sentences: After an initial period however distributors cease calling to people who do not purchase. And only call at selected customers, (customer base) so the initial stage of blanket dropping is used as a filtering process to find customers.