Using the “Re-dial” Command
1. Press and hold 
for two seconds.
2. After the tone, say “Re-dial.” Once connected, the person called will be heard through the audio speakers.
See also:
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Customer Assistance for Text Telephone (TTY) Users (U.S. and Canada)
To assist customers who are deaf, hard of hearing, or speech impaired and who use Text Telephones (TTYs), GMC has TTY equipment available at its Customer Assistance Center. Any TTY user can communicat ...
OnStar Overview
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The OnStar system statu ...