Willie the electrician receives a telephone call late at night from Colin. Colin explains that he has just come home and his house has no power. Colin insists that Willie come over immediately because it is the middle of winter and he is without heat. Willie explains that he charges extra for emergency calls, but no actual price is ever discussed over the phone. Willie arrives at Colin's house. He finds that the house has no power because Colin tripped the main circuit breaker by plugging in too many appliances. Willie simply turns the main circuit breaker back on and leaves. He sends a bill to Colin for $400.

Is Colin obliged to pay this amount? Explain.