Bell spokesman Paolo Pasquini said Monday that, as a precaution, the company is also contacting all other small business customers who receive similar services from Bell.
"We disabled all passwords when we learned of the breach and contacted credit card issuers, law enforcement and government security officials," Pasquini said in an email.
"We're still continuing the investigation with those authorities and our supplier," he said.
Bell has said 22,400 of its small business customers had their account information compromised by hackers.
Bell said the breach of usernames and passwords occurred when an Ottawa-based third-party supplier had its systems hacked and the information was then posted to the Internet on the weekend.
A group calling itself Nullcrew is claiming responsibility for the attack, and tweeted a link to the data early Saturday.
Bell has said five valid credit card numbers were included in the information dump.
But the company said on the weekend that its own systems were not hacked, and that its residential, mobility and enterprise customers are not affected.
Twitter postings by the Nullcrew account suggest the supplier's network may have been compromised nearly three weeks ago.
Bell has not said when the company became aware of the attack.Suggest a correction