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user_behavior = calls_per_month\
    .merge(messages_per_month, left_index=True, right_index=True, how='outer')\
    .merge(sessions_per_month, left_index=True, right_index=True, how='outer')\
    .merge(minutes_per_month, left_index=True, right_index=True, how='outer')\
    .reset_index()\
    .merge(users, how='left', left_on='user_id', right_on='user_id')\

user_behavior.head()