There are many ways you can achieve unified messaging functionality in Exchange 2013. It all depends on your Exchange, Lync and telephony infrastructure.
Before you begin, you have to install Exchange language pack for non-English Exchange deployment. For English deployment you don’t need to install language pack.
Depending on your Exchange 2013 version, Download Exchange Language Pack from the following web sites.
http://www.microsoft.com/en-au/download/details.aspx?id=35368
http://www.microsoft.com/en-au/download/details.aspx?id=39713
http://www.microsoft.com/en-au/download/details.aspx?id=41176
Right click the UMLanguagePack.Country-Code.exe file, Click Run As Administrator.
In the Exchange 2013 Setup wizard, on the License Agreement page, select I accept the terms in the license agreement, and then click Next then click Install.
Click Finish to complete the installation of the UM language pack.
Scenario#1
If you have a Cisco Call Manager for IP telephony then you just need to perform few tasks in Exchange 2013 to integrate Exchange and Cisco Call Manager. Here are the steps to accomplish unified messaging in Exchange 2013 with Cisco Call Manager.
Step1: Create a Service Account named domainnamesa-ExchangeUC and set password and account to be never expired. Set user cannot change password.
Step2: Open Exchange 2013 Management Shell as an administrator (Account must be a member of Exchange organisation management role). issue the following command.
New-ManagementRoleAssignment –Name:UMServicesConnectionACC –Role:ApplicationImpersonation -User:”domainanemsa-ExchangeUC “
Get-ManagementRoleAssignment
Step3: Create an anonymous relay in Exchange 2013. Here is a guideline.
Name: Anonymous Relay
Role: Frontend Transport
Type: Custom
Available IP: Exchange 2013 server IP
Port: 25
Authentication: TLS, Externally Secured
Permission: Exchange Servers, Anonymous users
Open Exchange Management Shell and execute the following
Get-ReceiveConnector “Anonymous Relay” | Add-ADPermission -User “NT AUTHORITYANONYMOUS LOGON” -ExtendedRights “Ms-Exch-SMTP-Accept-Any-Recipient”
Now add Cisco Call Manager IP address into remote network settings properties of anonymous relay.
Step4: Export Exchange Client Access Certificate from Exchange 2013 as .pfx format (public key included) and import into computer account of windows machine then export as .cer format certificate into Cisco Unity. reference http://www.digicert.com/ssl-support/pfx-import-export-iis.htm
Step5: Configure Cisco Unity for Unified Messaging. Follow this link to configure Cisco Call Manager. Detailed guide is available in Cisco Unity and Microsoft Exchange configuration guide.
Scenario#2
There are other ways to achieve same result if you decide Exchange 2013 to manage dial plan, auto attendant, hunt group and voice delivery etc. In this scenario, you have configure lot more then previous steps. There is no concrete steps for your scenario or your IP telephony systems. But here is what you have to do to accomplish unified messaging between IP-PBX and Exchange 2013. I assume your Exchange 2013 and IP-PBX are working per normal.
Step1: Create a Service Account named domainnamesa-ExchangeUC and set password and account to be never expired. Set user cannot change password in the properties of sa-ExchangeUC account.
Step2: Export Exchange Client Access Certificate from Exchange 2013 as .pfx format (public key included) and import into computer account of a windows machine then export as .cer format certificate into IP-PBX.
Step3: Configure IP-PBX to connect to Exchange 2013 using service account you have created in previous step.
Step4: Create a virtual extension number. This extension number will be used in a Exchange 2013 only.
Step5: Create a dial plan
In the Exchange admin center (EAC), navigate to Unified Messaging > UM dial plans, and then click Add 
.
On the New UM Dial Plan page, complete the following boxes:
Name: ExchangeUC Dial Plan
Extension Length: 4 or Exact length used in IP-PBX
Dial plan type: Telephone extension
VoIP security mode: Unsecured
Country/Region code: +61 (for australia)
Click Save.
Step6: Create a PIN Policy
In the EAC, navigate to Unified Messaging > UM dial plans. In the list view, click the ExchangeUC Dial Plan you have created in previous step and then click Edit 
.
On the UM Dial Plan page, under UM Mailbox Policies, select the UM mailbox policy you want to edit, and then click Edit .
Click Properties. On the UM mailbox policy page, click PIN policies.
On the PIN Policies page, configure the following PIN settings
PIN Length: 5
PIN Cycle: 5
Enforce PIN lifetime: 60
Sign-in failure: 5
Sign-in lockout:15
Click Save.
Step8: Add a DNS record in the forward lookup zone of Active Directory DNS
lets say DNS Name: IPPBX.domainname.com and corresponding IP: 10.10.70.240
Step8: Add UM IP Gateway
In the EAC, navigate to Unified Messaging > UM IP Gateways, and then click Add .
On the New UM IP gateway page, enter the following information:
Name: Cisco Unity or 3CX whichever is your gateway
Address: FQDN or IP Address of IP-PBX
UM Dial Plan: ExchangeUC Dial Plan
Click Save.
Step9: Create Auto Attendant
In the EAC, navigate to Unified Messaging > UM dial plans, select the ExchangeUC Dial Plan for which you want to add an auto attendant, and then click Edit .
On the UM Dial Plan page, under UM Auto Attendants, click Add .
On the New UM auto attendant page, complete the following boxes:
Name: ExchangeUC Auto Attendant
Uncheck “Create this auto attendant as enabled”
Uncheck “Set the auto attendant to respond to voice commands”
Access Number: click Add and add virtual extension number you have created in step 4.
Click Save.
Step 10 (Optional):
In the EAC, navigate to Unified Messaging > UM dial plans. In the list view, select the ExchangeUC Dial Plan and then click Edit .
On the UM Dial Plan page, under UM Hunt Groups, click Add .
On the New UM Hunt Group page, complete the following boxes:
Associated UM IP gateway: IPPBX.domainname.com
Name: ExchangeUC Hunt Group
Dial plan Click Browse to select the ExchangeUC Dial Plan
Pilot identifier: a string that uniquely identifies the pilot identifier obtained from IP-PBX.
Click Save.
Step11: Setup UM Dial Plan Policies
In the EAC, navigate to Unified Messaging > UM dial plans. In the list view, select the ExchangeUC Dial Plan and then click Edit .
On the UM Dial Plan page, under UM Mailbox Policies, click New .
On the New UM mailbox policy page, in the Name box, enter the name of ExchangeUC mailbox policy.
Click Save.
Step12: Enable User for Voice Mail
In the EAC, click Recipients. In the List view, select the user whose mailbox you want to enable for Unified Messaging.
In the Details pane, under Phone and Voice Features, click Enable.
On the Enable UM mailbox page, click the Browse and select ExchangeUC mailbox policy, and then click OK.
On the Enable UM mailbox page, complete the following boxes:
Extension Number: Type the extension number you have created in IP-PBX for this mailbox
PIN Settings: Type a 5 digit PIN number
Click Finish.
Now you have successfully configured Unified Messaging in Exchange 2013. However if you have Lync 2013 in your organisation. you will have to perform the following steps in Exchange 2013 to integrate Lync and Exchange.
Step1: Set Dial Start-up mode to dual
Open Exchange Management Shell, Enter the following command
Set-UmService -Identity “FQDN of Exchange Server” -DialPlans “ExchangeUC Dial Plan” -UMStartupMode “Dual”
Step2: Assign Exchange Certificate to UM
Type Get-ExchangeCertificate and copy the thumbprint in notepad
Enable-ExchangeCertificate -Server “FQDN of Exchange Server” -Thumbprint “EA5A332496CC05DA69B7578A110D22d” -Services “UM”
I assume that you already assigned this certificate to IIS, SMTP services. Restart the MsExchangeUM service on the Exchange server.
Step3: Assign certificate to call router
Set-UMCallRouterSettings -Server “FQDN of Exchange Server” -UMStartupMode “Dual” -DialPlans “ExchangeUC Dial Plan”
Enable-ExchangeCertificate -Server “FQDN of Exchange Server” -Thumbprint “45BAA32496CC891169B75B9811320F78A1075DDA” –Services “IIS”, “UMCallRouter”
Restart the MsExchangeUM service on the Exchange server.
Step4: Test UM Service
$credential = Get-Credential “DomainNameUser1”
Test-CsExUMConnectivity -TargetFqdn “FQDN of Exchange Server” -UserSipAddress “sip:User1@DomainName.com” -UserCredential $credential
$credential = Get-Credential “DomainNameUser2”
Test-CsExUMVoiceMail -TargetFqdn “FQDN of Exchange Server” -ReceiverSipAddress “sip:user1@DomainName.com” -SenderSipAddress “sip:user2@DomainName.com” -SenderCredential $credential
References:
http://technet.microsoft.com/en-us/library/jj673564%28v=exchg.150%29.aspx
http://technet.microsoft.com/en-us/library/jj150478%28v=exchg.150%29.aspx
Blog by Raihan Al-Beruni 