Scenario: When one has listed 200 leads/page and trying to send SMS selecting all lead records in one go, then one would come across this warning message.

Reason: From Manage leads page, it's not possible to send bulk SMS when 200 lead records listed / page.

Solution/ Workaround: One can send bulk SMS from Manage lead page only to 100 lead records/ page. If the lead records are more than 100 leads/ page, then its suggested to add all those lead records into a list and send bulk SMS.