People use MultiLine as their work number for communicating with clients, partners and co-workers. When it comes to internal communications between co-workers, it's important to keep the conversation happening between work numbers rather than any personal numbers that may be added to the Contact list.
The MultiLine Indicator "M" icon appears near a contact name or number to identify it as a MultiLine number. This makes it easy to identify a co-worker's MultiLine number within MultiLine applications. The indicator appears for all MultiLine users in the company across all regions where the company operates.
Note: If your admin disables MultiLine Indicator, you will not see these icons.