Change the AI ​​of entities in-game at the touch of a button so that they follow the player

Hey, 

I would like to make entities that heal the player when they are nearby. 

These should spawn randomly and follow the player (in a reeeaaally big radius) when he gives them a signal (in my case with a telephone). 

Here's the question: How can I make the entity follow the player in a huge radius when the player requests help (using the telephone device, as mentioned above)?

Thx.