When sending WhatsApp and SMS messages you need to be able to define the number that these are sent from.
This can be done at a Service level which acts as a defult.
These should be setup when creating the Message Service
The default can be overridden by a value entered into the Brand Settings, to allow different Send From numbers for each Brand.
To set this, within Brand Settings you can select Senders from the Navigation menu on the left:
Here you can define the SMS Number or WhatsApp Number and these will be used as the Send From number for these message types.
Lastly both of these settings can be overridden by a value entered into the Message Sender profile that can be set against a Message, to allow a different Send From number for a specific Message Sender (for instance you could set your Billing Send From number against your Billing Sender)
To define this, go to Organisation Settings, Messenger, Senders:
Click Edit against the Sender you want to send from, and enter the SMS number or WhatsApp number:
Save the sender with the Send From numbers entered.
When this Sender is set against an SMS or WhatsApp Message, the numbers in this Sender account will be used instead of any defined in the Brand Settings or Service Settings