Any wrecker service which is not on the rotation list and that is requested to respond to the scene of an accident by the owner or operator of a disabled or wrecked vehicle may respond when requested, so long as the wrecker responds within fifteen (15) minutes from the time it is notified. A city business license is not required of a wrecker service when there is nothing to indicate that the service was anything other than an isolated, incidental or casual one. Repeated services by a wrecker shall be considered as engaging in business and shall be required to obtain a business license.