The federal government says it can't measure how many people actually receive emergency-alert messages on their phones.
A spokesman for Public Safety Canada says the most recent test of the warning system in early May was determined to be a success.
Wireless providers reported a successful distribution of the test messages across their networks.
However Tim Warmington also says there is no mechanism for the providers to know how many phones actually received the message.
It became mandatory for wireless providers to issue emergency alerts on mobile phones in April 2018.
Concerns about the limitations of the system arose this week when numerous Ottawa residents reported never getting warned about a tornado that hit an east-end neighbourhood on Sunday night.