What type of sentence is while I was talking on the phone to Walter?

The phrase "while I was talking on the phone to Walter" is not a complete sentence. It's a dependent clause, also known as a subordinate clause.

Here's why:

* It contains a subject ("I") and a verb ("was talking").

* However, it does not express a complete thought. It needs an independent clause to make sense.

For example:

* While I was talking on the phone to Walter, the doorbell rang.

In this example, "while I was talking on the phone to Walter" is the dependent clause, and "the doorbell rang" is the independent clause. Together, they form a complete sentence.