using both database operation and external method in Unit of Work

I have to use a web service and update a table in database. Conventionally I perform these steps:

1. Open a transcation
2. Call db update method(There no risk If it is changed nothing because it will rollback)
3. Call a web service(external)
4. If web service results is 200(ok) then call commit else rollback

But unit of work has one method SaveChanges(). How can I do this scenario in unit of work?