If you install a
Cisco Unified CME system to replace an older telephony system that had an
established telephone number plan, you can retain the old number plan.
Cisco Unified CME supports flexible extension number lengths and can provide
automatic conversion between extension dialing and E.164 public telephone
When a router
receives a voice call, it selects an outbound dial peer by comparing the called
number (the full E.164 telephone number) in the call information with the
number configured as the destination pattern for the POTS dial peer. The router
then strips out the left-justified numbers corresponding to the destination
pattern matching the called number. If you have configured a prefix, the prefix
will be put in front of the remaining numbers, creating a dial string, which
the router will then dial. If all numbers in the destination pattern are
stripped-out, the user will receive (depending on the attached equipment) a
Cisco Unified CME system requires a telephone numbering plan that supports
future expansion. The numbering plan also must not overlap or conflict with
other numbers that are on the same VoIP network or are part of a centralized
voice mail system.
Cisco Unified CME supports shared lines and multiple lines configured with the same extension number. This means that you can set up several phones to share an extension number to provide coverage for that number. You can also assign several line buttons on a single phone to the same extension number to create a small hunt group.
If you are
configuring more than one Cisco Unified CME site, you need to decide how calls
between the sites will be handled. Calls between Cisco Unified CME phones can
be routed either through the PSTN or over VoIP. If you are routing calls over
VoIP, you must decide among the following three choices:
You can route
calls using a global pool of fixed-length extension numbers. For example, all
sites have unique extension numbers in the range 5000 to 5999, and routing is
managed by a gatekeeper. If you select this method, assign a subrange of
extension numbers to each site so that duplicate number assignment does not
result. You will have to keep careful records of which Cisco Unified CME system
is assigned which number range.
You can route
calls using a local extension number plus a special prefix for each
Cisco Unified CME site. This choice allows you to use the same extension
numbers at more than one site.
You can use an
E.164 PSTN phone number to route calls over VoIP between Cisco Unified CME
sites. In this case, intersite callers use the PSTN area code and local prefix
to route calls between Cisco Unified CME systems.
If you choose to
have a gatekeeper route calls among multiple Cisco Unified CME systems, you may
face additional restrictions on the extension number formats that you use. For
example, you might be able to register only PSTN-formatted numbers with the
gatekeeper. The gatekeeper might not allow the registration of duplicate
telephone numbers in different Cisco Unified CME systems, but you might be able
to overcome this limitation. Cisco Unified CME allows the selective
registration of either 2- to 5-digit extension numbers or 7- to 10-digit PSTN
numbers, so registering only PSTN numbers might prevent the gatekeeper from
sensing duplicate extensions.
Mapping of public
telephone numbers to internal extension numbers is not restricted to simple
truncation of the digit string. Digit substitutions can be made by defining
dial plan patterns to be matched. For information about dial plans, see
Dial Plan Patterns. More
sophisticated number manipulations can be managed with voice translation rules
and voice translation profiles, which are described in the
Voice Translation Rules and Profiles
In addition, your
selection of a numbering scheme for phones that can be directly dialed from the
PSTN is limited by your need to use the range of extensions that are assigned
to you by the telephone company that provides your connection to the PSTN. For
example, if your telephone company assigns you a range from 408 555-0100 to 408
555-0199, you may assign extension numbers only in the range 100 to 199 if
those extensions are going to have Direct Inward Dialing (DID) access. For more
information about DID, see
Direct Inward Dialing Trunk Lines.