Optus has been fined $110,000 for sending 20,000 electronic messages to its customers.
The fine is the second biggest penalty imposed for a breach of Australia's anti-spam laws.
The messages promoted the OptusZoo entertainment service to customer mobile phones and used the sender identification 966.
Optus assumed recipients would make the connection between 966 on a mobile phone keypad, and the word "ZOO" which can also be spelt out using the three numbers, the Australian Communications and Media Authority (ACMA) said.
"However, this was not considered sufficient identification, as '966' could be used to represent any number of permutations on a telephone keypad," ACMA chairman Chris Chapman said in a statement.
Negotiations between Optus and ACMA aimed at resolving the issue failed, resulting in the issue of infringement notices under the Spam Act.
AAP