If you have more than one MessageMedia account, you may want to assign a specific account to be used by default for particular Users. For example, you may have one account for Sales, one for Marketing and another for Customer Support.
To set a default account to a User, edit the User’s record and enter the MessageMedia Account Name into the ‘Mercury SMS Default Account’ field and click Save. Note: You may need to add the field to the User Page Layout. 
When a User has a Default Account set, and they go to send an SMS, this Account will automatically be selected by default.
The Default Account will also be used for messages using the SMS Feed, SMS Template and messages triggered via Workflow Rules / Process Builders.