More than 100,000 Telstra users are still using 3G-only devices and have until June 30 to upgrade their phone before Telstra switches off its 3G network.

The shutdown will also affect one million older 4G and 5G phones that use 3G to call Triple Zero.

Popular devices impacted by the switch off include iPhone models older than the iPhone 5s, older iPad models, and the Samsung Galaxy S5.

The shutdown comes 20 years after 3G was installed and Telstra said it was "repurposing it to expand our faster 5G network".

"If a customer will be impacted by the shutdown, they will likely have already heard from Telstra," the telco said.

Any Telstra network user could text '3' to 3498 and they would receive a reply telling them of any action needed.

The message would inform users that "it's time to act" or "you don't have to do anything".

If "it's time to act", customers must upgrade to a 4G or 5G device before service is cut in June.

Concerns have also been raised about other devices that still relied on 3G.

Better Internet for Rural, Regional and Remote Australia (BIRRR) spokesperson Sharon Single said members were concerned about device coverage.

"Many of the products used especially in rural and remote areas, such as boosting equipment, ag tech devices, EFTPOS machines, old style mobiles, they're likely to reach end of life," she said.

Telstra is encouraging medical alert device users to check with manufacturers to ensure their devices is 4G VoLTE.

Optus will switch off its 3G network in September.