38 CFR § 17.54(a)

The admission of a veteran to a non-Department of Veterans Affairs hospital at Department of Veterans Affairs expense must be authorized in advance. In the case of an emergency which existed at the time of admission, an authorization may be deemed a prior authorization if an application, whether formal or informal, by telephone, telegraph or other communication, made by the veteran or by others in his or her behalf is dispatched to the Department of Veterans Affairs (1) for veterans in the 48 contiguous States and Puerto Rico, within 72 hours after the hour of admission, including in the computation of time Saturday, Sunday and holidays, or (2) for veterans in a noncontiguous State, territory or possession of the United States (not including Puerto Rico) if facilities for dispatch of application as described in this section are not available within the 72-hour period, provided the application was filed within 72 hours after facilities became available.