She did know his phone number, otherwise she would phone / would have phoned him.

didn't, didn't know his phone number, otherwise she'd have phoned him.

either sentence works with didn't, depending on the tense.

1. She didn't know his phone number, otherwise she would have phoned him.

2. She didn't know his phone number, otherwise, she would phone him.

Number 1 definitely sounds better [note the change to the negative "didn't". I can see #2 as a possibility if the fact that she still doesn't know the phone number is known to the parties in the conversation.

A: She doesn't know his phone number?

B: Nope. She didn't know his phone number then either - otherwise, she would phone him [right now].

Thanks, Wayne.

Thanks, JTT, for pointing out the missing word: 'not'. It was inadvertently left out. Also thanks for the explanation.