Link Access Procedure on the D channel. This is a data link layer protocol specified in ITU-T Q.921, used to carry signaling and control information across an Integrated Services Digital Network. It ensures that call setup, teardown, and routing messages are delivered reliably and in the correct sequence between the subscriber equipment and the telephone exchange.
We're seeing LAPD frame drops on the primary rate interface, which explains why the PBX is failing to establish calls.
Related terms
ISDN
Integrated Services Digital Network is an older standard for transmitting digital voice and data over standard telephone lines.
PRI
The primary rate interface is an enterprise telecommunications standard for carrying multiple voice and data channels.
Primary rate interface
An enterprise-grade ISDN connection that bundles multiple digital voice and data channels over a single physical line.