Instructions
Call your homie on the in-game telephone. Many times the homie will answer and the game can proceed. Sometimes, however, the homie will opt not to answer the phone or will tell you he is not available.
Wait a few minutes and try calling the homie again. Occasionally, a pause is enough to allow the homie to change his mind and either answer the phone or give you a different response.
If you are inside a building or home, have your character walk outside. This can sometimes resolve the situation. In some instances, however, the homie is more stubborn.
Complete your current mission without the homie. In some cases, you will need to get past a certain point in the mission before the homie will cooperate.