On July 16, 2020 the FCC approved a 3 digit dialing code (988) to be used as the new National Suicide Prevention and Mental Health Crisis hotline. The new hotline will be available starting July 16, 2022. Calls to 988 will reach the existing National Suicide Prevention Lifeline available by calling 1-800-273-8255 (1-800-273 -TALK)
In order to be able to activate the new 988 short dialing code, area codes (numbering plan areas) using both 7 digit dialing and 988 as a central office code are required to transition to 10 digit dialing by July 16, 2022.
See this official document for a detailed history of the Implementation of the National Suicide Hotline (PDF document)