This article applies to:
You use Exchange Unified Messaging for voicemail and your Skype clients are used to retreive voicemail. In between your clients and the Exchange server you have a firewall and your firewall administrator is absolutely not delighted with the idea to open the entire port range of 1025-65535 for RTP protocol, which Exchange UM uses by default to deliver the media streams. The firewall administrator requires you to narrow the range to a smaller one, let’s say 49152-59964
You can set the Minimum and Maximum RTP port for your Exchange UM in the MSExchangeUM.config file. The file is located in the Exchange binary folder typically under C:\Program Files\Microsoft\Exchange Server\V15\Bin\MSExchangeUM.config
Replace the default values with your own ones and restart the Exchange UM and Exchange UM Call Router Services.
IMPORTANT: make sure you remove the trailing spaces (marked red) otherwise the Exchange Server will treat those as invalid entries and will use the default vales of 1024-65535.
For further information on protocols, ports and services used by Exchange Unified Messaging check the official Microsoft documentation under: Exchange UM Protocols, Ports and Services