Configuring Cisco Unified CME to use H.450.2 transfers (this is recommended), set transfer-system full-consult under the telephony-service command mode. We also need to use this configuration for SIP VoIP transfers. There are two types of transfer modes configured on Cisco Unified CME router, these are covered below.
Consult – it allows a call transfer after talking to the calling party. Once you dial the number to which you want call to be transferred, you can wait for the other party to answer and speak with them before doing the call transfer. Pressing the trnsfer softkey second time will transfer the call and drop you out of conversation. This kind of transfer requires a second line(or dual-line configuration) and this is the default mode of the CME router.
Blind – in this mode call is transferred immediately after you dial the number. It works in single line configuration.
To configure which transfer mode to be used, use the following syntax:
CME_Voice (Config) # telephony-service
CME_Voice (Config-telephony) # transfer-system?
Full blind performs call transfer with consulting using the H.450.2 or SIP Refer standard methods.
Full-consult performs H.450.2/SIP call transfers with consultation using a second phone line if available and falls back to full-blind if the second line is unavailable. This is the recommended mode for most of the systems. See also supplementary-service command under voice service setup and dial-peer
Local-consult performs call transfers with local consultation using second phone line if it is available. It will fallback to blind for non-local consultation or if the transfer target uses a Cisco proprietary method.
CME_Voice (Config-telephony) # transfer-system full-consult
The full-blind and full-consult use H.450.2 which an industry standard method of call transferring. Using the H.450.2 standard while doing a call transfer, the CME router drops the call completely from the transferred system and initiates a new call on the phone to which call is transferred.
The local-consult method use Cisco proprietary transfer method which will perform a consult transfer when multiple or dual line configurations are available; otherwise blind transfer will be used when a single line is available. The only issue with Cisco proprietary method of call transfer is resulting in hairpinned calls when you have non-Cisco telephony systems in the network.
We can also configure transform modes selectively for each ephone-dn using transfer-mode syntax from ephone-dn configuration mode. By default transfer mode configuration uses the H.450 standard and overrides any other system wide setting.
For devices which cannot be managed locally, Cisco by default does not do transfer to them. This is just to avoid toll frauds. In case you need to allow transfers outside of the locally managed devices, we can use the transfer-pattern command from telephony service mode where the pattern is representing numbers to which we would like to allow transfers. In the below example, we will configure the Cisco Unified CME router to allow transfer to 5XXX and local ten-digit PSTN number.
CME_Voice (Config) # telephony-service
CME_Voice (Config-telephony) # transfer-pattern?
WORD digit string pattern for permitted non-local call transfers
CME_Voice (Config-telephony) # transfer pattern 5…
CME_Voice (Config-telephony) # transfer pattern 9…
The transfer-pattern command also allows specifying that specific transfer-to destinations should receive only blind transfers. We also have to use this configuration for SIP VoIP transfers. The transfer-pattern command allows you to restrict trunk-to-trunk transfers to prevent incoming PSTN calls from being transferred back out to the PSTN (employee toll fraud). Trunk-to-trunk transfers are disabled by default, because the default is to allow only local extension-to-extension transfers.
The supplementary-service command is used to allow H.450.12 service to automatically detect the H.450.2 capabilities of endpoints in your H.323 VoIP network.
To enable hairpin routing of VoIP calls that can’t be transferred (or forwarded) using H.450, use the allow-connections command. The example below shows a call transfer configuration using this command.
Voice service voip
Allow-connections h323 to h323
The configuration in the above example turns on the H.450.2 (transfer-system full-consult) and H.450.12 services, allows VoIP-to-VoIP hairpin call routing (allow-connections) for calls that don’t support H.450, and permits transfers to all possible destinations (transfer-pattern). The transfer permission is set to provide full wildcard matching for any number of digits. (The T stands for terminating the transfer destination digit entry with a timeout.)
For configuring a more restrictive transfer permissions see the example below:
Transfer-pattern 2… Blind
The above configuration permits transfers using full consultation to nonlocal extensions in the range 1000 to 1999. It also permits blind transfers to nonlocal extensions in the range 2000 to 2999.
This concludes lesson on call transfer from a CCNA Voice perspective. In this lesson we learned about call transfer, the types of call transfer and how to configure call transfer options.