You have to unlock Dwayne by not killing him and then replaying to an email from him. After that You can call him.

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After you complete the mission The Holland Play in Grand Theft Auto IV, Dwayne Forge is unlocked as friend (if you chose not to kill him). This is accomplished only after you reply to the email he sends you. After he is unlocked, he will occasionally call you and ask if you want to engage in a variety of activities around Liberty City. The more you do things that Dwayne likes, the more his “like” percentage will rise (viewable from the stats screen in the menu). When his “like” percentage reaches 60% his special bonus ability will be unlocked: when you call Dwayne he will offer to send a couple armed NPCs to help protect you on missions, a very useful ability. The raise Dwayne's “like” percenage faster, engage in activities that he prefers: Cluckin' Bell, or going to Club Liberty in Northwood.
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Source(s):
http://www.mahalo.com/Grand_Theft_Auto_4_Dwayne_Bonus

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