Where to find it
Go to Audience → Contacts → Merge candidates. The page has two tabs: Merge candidates and Merge history.
Review a suggestion, step by step
What happens when you merge
Undo a merge within 30 days
Automatic merges
When the match is certain, Mersal merges for you. Certain means exactly one contact has that phone number and nothing conflicts. Automatic merges appear in Merge history, and you can undo them the same way.