Also (with all the long distance companies and a lot of companies with huge call centers) call different times and days. Different skill levels work different days and hours due to their seniority usually. So on a Wednesday daytime you hopefully get a full time experienced person, where on Saturday afternoon of Memorial Day weekend you may be more likely to get the new person.
Of course there are exceptions because they do try to balance things but the odds are...
You can use either scenario to your advantage.

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