Using the “Dial” or “Call” Command
1. Press and hold 
for two seconds.
2. Say “Dial” or “Call.” 3. Say the entire number without pausing, or say the name tag.
Once connected, the person called will be heard through the audio speakers.
See also:
Maintenance Replacement Parts
Replacement parts identified below by name, part number, or specification can
be obtained from your dealer.
...
2008 GMC Yukon Review
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Power Liftgate
On vehicles with a power liftgate, the vehicle must be in P (Park) to operate. The taillamps flash and a chime sounds when the power liftgate moves.
WARNING
You or others could be injured if caught ...