As claimed on https://direct.starlink.com/:

Direct to Cell works with existing LTE phones wherever you can see the sky. No changes to hardware, firmware, or special apps are required, providing seamless access to text, voice, and data. 

"Existing LTE phone" means a mobile phone commercially available in at least one market not after December 31st, 2023.

"Offer" means a public or beta launch available to at least 100 members of the general public (i.e. not limited to journalists, SpaceX employees) in at least one country. Requiring a new or existing subscription with a specific network counts (e.g. T-Mobile in the US).

"No changes to hardware, firmware, or special apps are required" will be judged by a specific mention of Starlink, SpaceX, or LTE-based satellite service in any software or firmware update released by the manufacturer between the launch of the device and the end of 2024. Non-Starlink or non-LTE satellite services, such as Apple's satellite-based emergency SOS service, can be mentioned.

"Text" means bidirectional texting using the phone's built-in messaging client (SMS or anything else the phone supports without installing an application not already present on the device as a factory default) with another ground- or space-based mobile phone and can't be limited only to SOS applications.