This code system http://hl7.org/fhir/service-referral-method defines the following codes:

CodeDisplayDefinition
faxFaxReferrals may be accepted by fax.
phonePhoneReferrals may be accepted over the phone from a practitioner.
elecSecure MessagingReferrals may be accepted via a secure messaging system. To determine the types of secure messaging systems supported, refer to the identifiers collection. Callers will need to understand the specific identifier system used to know that they are able to transmit messages.
semailSecure EmailReferrals may be accepted via a secure email. To send please encrypt with the services public key.
mailMailReferrals may be accepted via regular postage (or hand delivered).