As part of the v2-to-FHIR project we are mapping commonly used fields in key v2 messages to FHIR resources so that messages can be transformed into bundle of FHIR resources. OBR-17 is the Order Callback Phone Number field but there is no cognate existing data element in FHIR. We suggest that a standard extension to ServiceRequest be created to allow exchange of this concept. 

While ServiceRequest does have a .requester field which may contain a Practitioner, PractitionerRole or Organization reference, but the entity that placed the order may not be the best point of contact for follow up questions or to call with the results (eg. a provider may place an order for an admitted patient, but it's the nursing station or another provider that could answer follow up questions). It's important to have a discrete place to provide the contact info.

We suggest a ContactPoint data type. 

The v2 description can be reused as part of the extension narrative:

This extension contains the telephone number for reporting a status or a result. (we may want to include other use cases too)