Under SRST, you can still benefit of the voicemail system integration at the central site but this time instead to have an internal call to Unity, you need to make an outside calls to Unity when SRST is enbale.
So under the call-manager-fallback command, enable the voicemail <<VM_number_to_dial>> and your end-users will still have the opportunity to press the message button in SRST
Survivable Remote Site Telephony (SRST) is a feature which ensures that IP Phones can continue to function even if they are unable to communicate with Call Manager. During a failure, Cisco IP Phones register with the local SRST router which provides Call Processing and Control.
With the Connection Monitor Duration, Cisco IP Phones do not fail back immediately to ensure that the Call Manager in question is back online and is stable.
Here is also another presentation of Cisco SRST ( Cisco video)
A dialing domain is a group of Unity servers that share the same directory and do not require subscribers to use any type of prefix when performing transfers and sending messages between the servers (Extension must be unique within the dialing domain).
Unity can also integrate with other differents types of voice mail systems and is referred to as Unity networking.There are two overall categories of Unity networking:
Networking with other Unity systems : This can be implemented using digital networking, SMTP networking or Voice Profile for Internet Mail (VPIMv2) networking depending on Unity version and specifications.
Networking with traditional/Non-Unity Systems : This can be implemented using a bridge networking, an Audio Messaging Interchange Specifications (AMIS) networking or a VPIM networking.
The specific methods of networking Unity can be described as follows:
Digital networking : This type of networking can be used to integrate Unity systems that share the same directory.
SMTP networking : This can be used to integrate Unity systems that do not share a directory, but that are connected via IP.
VPIM networking : This allows integrations of Unity with either other Unity systems or Non-Unity systems over an IP network such as the Internet. In this case of Unity-Unity Systems integration , it is obvious that the whole set can not share the same directory (AD forest).
Bridge networking : This can be used to integrate Unity with an Octel voice mail system and requires the use of a Bridge server.
AMIS networking : This allows Unity to integrate with Non-Unity systems over analog lines.
Take in mind that VPIM is a standard which is used by messaging systems to communicate messages over the Internet or other IP networks.
VPIM is based on the SMTP and Multipurpose Internet Mail Extension (MIMI) protocols. Because SMTP transport is used with VPIM, it is essential that the Exchange server used by a local Unity server, must be able to exchange email with the email system that the remote voice mail system is using.
When voice messages are sent using VPIM, they are typically converted into G726 (for Non-Unity systems) .For fax messages , they are encoded using a TIFF-F format.
Before messages can be sent between VPIM systems, they must be encoded as MIME messages, and with Unity this task is handled by the component called the Cisco Unity Voice Connector for Exchange (referred as Internet Voice Connector (IVC)). The IVC is also responsible for handling incoming VPIM messages in conjunction with the component called the VPIM Transport Event Sink.
Don’t forget also before to configure VPIM networking on Unity that you must complete several preliminary tasks as:
Extending the AD schema
Installing the IVC
Extending the AD schema is also a mandatory step to perform for Bridge networking.
Unity can be enhanced to provide voice mail, mail, fax , text-to-speech, and so on. So in order for Unity to provide a full UM integration , Unity must be interfaced with several elements including a directory , a mail store, IIS and SQL server.
So with all theses , there are 2 types of possible integrations:
If the mail store is provided by Microsoft Exchange, Unity also integrates with Microsoft Active Directory (AD). Then it is required that Unity makes a schema extension to the AD domain, this extension will make grow the AD by up to 10 percent.Here are the Unity extensions:
Unity location (this class is created)
An alternative to using Microsoft Exchange as the message store, you can use also Lotus Notes. When Lotus Notes is used as message store, it is required also to implement the module called Domino Unified Communication Service (DUCS).