The site prefix identifies the remote site in the network numbering
plan.
The site prefix is not optional.
The site prefix can be from one to nine digits. Make the site prefix the same as the ESN code or tie-line number for the remote site as it appears in the local telephony dialing plan so that it is easy to remember.
The prefix must be unique for the local site, and it cannot overlap with any other prefix in the table. For example, the prefix 2345 overlaps with prefixes 234 and 23456.