If the modem supports @, it might be easier to deal with waiting for an answer. (and I don't like m0 for automated processes, as I want to know that the modem's making a call.)

my $phone_number = '555-1111';
my $message      = '911';

# assuming TTY like above
print TTY 'atdt',$phone_number,'@1,',$message,"@;\rath\r";
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(and my mom thought BBSing was useless)