Telstra and Optus users across Australia are reporting issues with mobile connection including phone calls, as a large number of people work from home.
There may also be longer delays in contacting customer service departments, many of which are based overseas, and dealing with stricter COVID-19 restrictions.
Telstra says its teams in Australia and India are online, but calls from customers in urgent need of help will be prioritised.
There is no impact to emergency 000 calls.
While stores remain open, Telstra and Optus are urging customers to use websites and apps where possible.
Telstra announced it will recruit 1,000 more temporary customer agents in Australia to respond to customer service requests.