In my version of Excel (Excel 2003) the maximum rows allowed in 65,536.
(I've seen references to greater amounts in subsequent or future
versions, but it sounds like you're using this version or something
similar as well.) Since the do not call list is likely to be pretty
large, in terms of a row count, you'll either need to expand into
different columns, or spread it across multiple tabs in the workbook,
or use a database, which would have no such row restrictions.

What do you need to do with the data?