CRA considers that national arts service organizations generally are NPOs rather than charities, and that on-call services are not services
CRA considers that national arts service organizations which register with it do not qualify as charities for GST/HST (or presumably ITA) purposes – although they generally will qualify as non-profit organizations if they are not operated for profit.
A medical facility may pay a doctor set fees for agreeing to be on call for specified periods. CRA has published its view (confirming 8 January 2016 Interpretation 150125) that such fees not only are not for exempt health services, but constitute consideration for supplies by the doctor of intangible personal property. (Agreeing to not engage in conflicting activities is not a service?)