Board Thread:Questions and Answers/@comment-39677734-20190602182242/@comment-34974603-20190606042940

The requirements for receiving the call from Undyne are:

- You have not killed any monster.

- You have gone on a date with Papyrus.

- You have spent time with Undyne.

- You have been judged by Sans as a Pacifist. (I think...)

- You have completed the Neutral Route with the above requirements met. (I think...)

After doing all of that, then she "should" receive the call when she exits the CORE. If she didn't, then she should try going to see Undyne anyway. Undyne may still give her the letter.

Also, when I say I think, I mean that that's how I received the True Pacifist ending, but I don't know if it was necessarily "required."