The sequence of events involved in receiving a Priority Service priority level assignment is as follows:
A Service User submits a request to their Authorizing Agent for the lowest applicable priority level necessary.
The Authorizing Agent forwards the request with a recommendation for approval or denial to the national designated authority.
The National Designated Authority returns the final priority level assignment to the Authorizing Agent as well as provides the Service User information (including assigned priority level) to the Service Provider.
The Authorizing Agent forwards the final priority assignment information to the Service User to complete the feedback loop.
for descriptions of Authorizing Agent and National Designated Authority.
Within U.S. networks, a Priority Service call is invoked by dialling *SC + Destination Number, with a Service Code (SC) of "272"
Within U.S. networks, a Service User is assigned one or more Access Classes in the range of 12 - 14 to receive priority access to the network, in addition to an assigned Access Class in the range of 0-9.
Within the U.S., there are five Priority Service priority levels. Table A.1
indicates the relationship among Service User Priority Assignment, TS 22.011
Access Classes, TS 22.067
eMLPP Priority Level, TS 48.008
Priority Level, TS 48.008
queuing allowed (qa) Value, and Precedence Level in the ISUP Precedence Parameter for Priority Service.
Priority assignment for ISUP messages is specified in ANSI T1.111.5, Annex A 
. For a Priority Service call, MTP message priority of 1 is used for ISUP IAM messages signalled to establish the call.
A Priority Service call invokes the High Probability of Completion (HPC) Network Capability as described in T1.631 
. The HPC Network Capability sets Calling Party's Category parameter to "NS/EP call"
in the ISUP IAM message to indicate that the call has been authorized to receive priority treatment. In the U.S., Service Providers use the "NS/EP call"
value and not the IEPS value. For a Priority Service call, the Calling Party's Category parameter value of "NS/EP call"
is propagated through the network and signalled to any interconnected network.
Service Provider transit nodes are expected to receive and pass the ISUP IAM message unaltered for all Priority Service calls. This explicitly includes the Calling Party's Category (CPC) parameter, the MTP Priority of the ISUP IAM message, and the ISDN MLPP Precedence Parameter.
For a Priority Service call, the ISUP Precedence parameter is encoded as indicated in Table A.2
The Precedence parameter includes the Look Ahead for Busy field to indicate whether a TCAP message may be sent prior to call setup to determine whether the called party's line is busy. Recognizing the call setup may be delayed, Priority Service does not support this functionality and the Precedence parameter is encoded accordingly.
The Network Identity (NI) field identifies the network or country which administers the service, not the country from which the call originates. Each digit is coded in binary coded decimal representation from 0 to 9. The first digit of this field is coded 0. The TCC (Telephone Country Code) follows in the second to fourth NI digits (the most significant TCC digit is in the 2nd NI digit). If the TCC is one or two digits long, the excess digit(s) is inserted with the code for the Recognized Operating Agency (ROA) or network identification, if necessary. If the 3rd and 4th NI digits are not required, they are coded all zeros.
The MLPP Service Domain is a 3-octet subfield that carries a code expressing in binary representation the number allocated to an MLPP Service Domain. These numbers are allocated in accordance with the procedures in Annex B, T1.113.3 
. The MLPP Service Domain values assigned to the National Communications System (NCS), the National Designated Authority as named in 
, correspond to Service User priority, i.e., NCS Wireless Priority Service-1 is used for a Service User priority of 1, etc. Therefore, there is no MLPP service interaction among the various levels of Service User priority nor between Priority Service and other services using the Precedence parameter.
For a Priority Service call, it is desirable that all TCAP messages sent during initial call setup associated with the Priority Service call are sent with MTP message priority 2.
Priority Service in the U.S. and U.S. territories is restricted to authorized individuals. Therefore, Service Providers do not implement Priority Service in their networks without an arrangement with the National Designated Authority. To support this restriction, the MSC/VLR should provide the capability to control access to, and activation of, the Priority Services software in the GSM BSS, MSC/VLR and HLR.