What it does

When enabled for a subscriber, the voicemail forwarding service can forward an incoming call to a voicemail server. Depending on the selected configuration, this can happen under any of these conditions:

  • The call has waited longer than a specified timeout for a successful response.

  • The called subscriber is unavailable and has no provisioned diversion rules for the MMTel call forwarding service.

  • The called subscriber has provisioned diversion rules, but none of them cause the call to be forwarded.

  • The called subscriber uses online charging and credit cannot be allocated.

For details about diversion rules, see Communications Diversion (CDIV).

The URI of the voicemail server to forward to is provisioned per subscriber. It is possible to allow a subscriber to choose their own voicemail server.

The voicemail forwarding configuration includes a list of URIs for known voicemail servers on the network.

Important If a subscriber’s chosen voicemail server does not appear in this list, it can change which announcement they will hear before the call is forwarded. It can also potentially disable voicemail forwarding when credit cannot be allocated.

The voicemail forwarding service only runs on the terminating instance of the Rhino VoLTE TAS, and can be triggered at any time during call setup.

Voicemail forwarding can be used with the Metaswitch Voicemail Server. See the VMS product documentation for details of how to set up the server.

Playing announcements

The Rhino VoLTE TAS supports playing an operator specified announcement to the calling party before forwarding to voicemail. If a subscriber’s voicemail server is on the list of known voicemail servers, the announcement can be specified for voicemail forwarding. If not, the standard Communications Diversion (CDIV) announcement will play instead.

These announcements are distinct from any announcement that the voicemail server itself might play to the calling party.

Forwarding to voicemail without credit

If the subscriber is using Diameter Ro based online charging and they will be charged for the incoming call, forwarding to voicemail can be invoked if credit could not be allocated. The policy for when this is allowed is part of the Voicemail Forwarding configuration.

Voicemail service codes

The Rhino VoLTE TAS supports a subscriber dialing a service code (for example *123) to retrieve voicemail by forwarding the originating call to their configured voicemail server. This function uses the same voicemail server URI in subscriber config as the voicemail forwarding service, but it runs on the originating Rhino VoLTE TAS instance.

There is also support for setting up a service code that a subscriber can use to set their voicemail server URI (if they are permitted to do so).

Interactions with other services

Call barring

The MMTel call barring services (ICB and OCB) always take precedence over voicemail forwarding. If barring is triggered, the call will be terminated without forwarding to voicemail.

Communications Diversion (CDIV)

Voicemail forwarding is closely related to the MMTel CDIV service. There are certain situations that can trigger either service, for example, if the called subscriber is already on a call. In such cases MMTel call forwarding takes precedence, and will evaluate the provisioned diversion rules for the called subscriber. Voicemail forwarding will be triggered if there are no diversion rules that result in the call being forwarded.

If a subscriber is allowed to provision their own diversion rules for the MMTel call forwarding service, they can use that functionality to forward the call to their own voicemail server. This is true even when they are not permitted to choose their own voicemail server for the voicemail forwarding service.

Important There are certain situations where the MMTel CDIV service may be suppressed, for example, when routing to a CS network where MSC is expected to handle call forwarding. In these situations the voicemail forwarding service will also be suppressed.

Online charging

When using online charging, voicemail forwarding can be configured to activate when credit cannot be allocated for the call (for any reason). If this behavior is triggered, voicemail forwarding will finalize the online charging session. After this, there will be no further communication with the online charging system.


What you need

  • ❏ The wait time for a call to be successfully connected before it is forwarded to voicemail.

  • ❏ A list of URIs for known voicemail servers on the network.

  • ❏ Whether to allow forwarding to voicemail if credit cannot be allocated for a subscriber that uses online charging. If allowed, forwarding can either be restricted to URIs on the known voicemail servers list or permitted for any URI.

  • ❏ Optionally, the ID of the announcement to be played before forwarding to any of the known voicemail servers.

  • ❏ Subscriber configuration set up with the Metaswitch-TAS-Services section enabled and the voicemail server URI field populated.

Setting up announcements

Voicemail forwarding configuration only needs the announcement ID of the announcement that should be played. The details of that announcement should be configured beforehand. For details about how to do this, see Announcements.

Setting up subscriber configuration

All subscribers using the Voicemail Forwarding service need to be configured with a voicemail server URI in the Metaswitch-TAS-Services section of their subscriber profile. Further information on configuring the subscriber data for the voicemail forwarding service is given in Forward to voicemail subscriber data.


For standard Rhino VoLTE TAS deployments, the Metaswitch-TAS-Services document is disabled. This must be enabled to activate voicemail forwarding using subscriber data.

Setting up service codes

The Rhino VoLTE TAS standard deployment includes service code actions for subscribers to retrieve voicemail and update their voicemail server URI (if permitted to do so).

Table 1. Supplied service code actions
Display name Action

Connect Subscriber To Voicemail Provider

Execute dialing for voicemail.

Set Default Forward Voicemail Server Number

Update the default forward voicemail number for the subscriber data (MetaswitchForwardToVoicemail.voicemailServer).

For more information, see Vertical service codes.

Setting up voicemail forwarding

The example for sentinel-volte-gsm-config.yaml and example for sentinel-volte-cdma-config.yaml show example configuration relevant to voicemail forwarding in the sentinel-volte/mmtel/call-diversion/forward-to-voicemail section.

I want to…​

Activate voicemail forwarding

Ensure the entire forward-to-voicemail section is present and not commented out in the configuration file.

                    - sip:vms1@example.com
                    - sip:vms2@example.com
                forward-to-voicemail-timeout-seconds: 0
                forward-to-voicemail-without-ocs-credit: NEVER_ALLOW
Set time to forward calls to voicemail after processing the initial INVITE
                forward-to-voicemail-timeout-seconds: 30
Disable forwarding to voicemail due to timeout
                forward-to-voicemail-timeout-seconds: 0
Disallow forwarding to voicemail if unable to allocate credit to the subscriber
Important This only applies to subscribers using Diameter Ro based online charging.

In the forward-to-voicemail section, set forward-to-voicemail-without-ocs-credit to never allow forwarding:

                forward-to-voicemail-without-ocs-credit: NEVER_ALLOW
Allow forwarding to voicemail regardless of being able to allocate credit to subscriber
Important This only applies to subscribers using Diameter Ro based online charging.

In the forward-to-voicemail section, set forward-to-voicemail-without-ocs-credit to always allow forwarding:

                forward-to-voicemail-without-ocs-credit: ALWAYS_ALLOW
Allow forwarding to a known voicemail server regardless of the credit status of a subscriber using online charging
Important This only applies to subscribers using Diameter Ro based online charging.

In the forward-to-voicemail section, set the following:

                forward-to-voicemail-without-ocs-credit: ALLOW_ONLY_FOR_WELL_KNOWN_SERVERS
                    - sip:vms1@example.com
Play a voicemail announcement (announcement id=7) to the calling party when voicemail forwarding occurs
                    - sip:vms1@example.com
                    voicemail-announcement-id: 7
Play the default forwarding announcement to the calling party when voicemail forwarding occurs

In the call-diversion, announcement section, remove or comment out voicemail-announcement-id:

                    # voicemail-announcement-id: 7
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Rhino VoLTE TAS Version 4.1