Availability Options:
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PSTN backup: Use the PSTN as a backup for on-net intersite calls.
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MGCP fallback: Configure an MGCP gateway to fall back, and use the locally configured (POTS), H.323 or (SIP) dial peers when the connection to its call agent is lost.
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SRST: Cisco IP Phones using either SIP or SCCP must register to a call-processing device for the phones to work.
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Call Forward Unregistered (CFUR): This is a call-forwarding configuration for CUCM that becomes effective when the IP Phone is not registered.
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Automated Alternate Routing (AAR) and
Call Forward on No Bandwidth (CFNB): AAR allows calls to be rerouted over the PSTN when calls over the IP WAN are not admitted by CAC. CFNB is a call-forwarding configuration of IP Phones, which becomes effective when AAR is used.
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Mobility solutions: CUCM Extension Mobility and the Device Mobility features.
SRST:
1. Define the SRST references for phones in CUCM Administration.
2. Configure the Call Forward Unregistered (CFUR) feature, and set the CFUR destination of lines on remote-site phones to the correct public switched telephone network (PSTN) number in CUCM Administration to enable reachable remote sites in SRST mode.
3. Apply to the Device Pool.
4. Configure SRST on gateway:
(config)#call-manager-fallback
(config-cm-fallback )#i p source-address ip-address [port
port] [any-match | strict-match]
(config-cm-fallback )#max-dn max-directory-numbers [dual-line] [preference
preference-order]
(config-cm-fallback)#max-ephones
max-phones
Optional:
(config-cm-fallback)#limit-dn {7910 | 7935 | 7940 | 7960}
max-lines
(config-cm-fallback)#keepalive seconds
5. Implement a simplified SRST dial plan on the remote-site gateways to ensure connectivity for remote-site phones in SRST mode.
MGCP Fallback:
(config)#ccm-manager fallback-mgcp
(config)#application
(config-app)#global
(config-app-global)#service alternate Default
(config)#voice service voip
(conf-voi-serv )#h323
--To provide call preservation during switchback
(conf-serv-h323)#no h225 timeout keepalive
CCME in SRST Mode:
(config)#telephony-service
(config-telephony )#srst mode auto-provision all
(config-telephony )#srst dn line-mode dual
(config-telephony )#srst ephone template 1
(config-telephony )#srst dn template 3
(config-telephony )#srst ephone description CCME SRST
(config-telephony)#max-ephone 10
(config-telephony)#max-dn 30
(config)#ephone-template 1
(config-ephone-template)#keep-conference local-only
(config)#ephone-dn-template 3
(config-ephone-template)#hold-alert 25 idle
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