The Free On-line Dictionary of Computing (30 December 2018):

Primary Rate Interface

   (PRI) A type of ISDN connection.  In North America and
   Japan, this consists of 24 channels, usually divided into 23 B
   channels and 1 D channel, and runs over the same physical
   interface as T1.  Elsewhere the PRI has 31 user channels,
   usually divided into 30 B channels and 1 D channel and is
   based on the E1 interface.

   PRI is typically used for connections such as one between a
   PBX (private branch exchange, a telephone exchange operated by
   the customer of a telephone company) and a CO (central office,
   of the telephone company) or IXC (inter exchange carrier, a
   long distance telephone company).

   (1995-01-18)