Should the guardian of a girl who is engaged in an illicit phone relationship be informed of what she is doing, or is it sufficient for the advisor to have offered advice, knowing that the guardian's reaction is unknown?

If you advise a girl to stop talking to a non-mahram man and she does not respond, then you should threaten her by telling her guardian. If she is still undeterred, then inform him, for the Messenger of Allah, peace and blessings be upon him, said: "Whoever among you sees an evil, let him change it with his hand; if he is not able, then with his tongue; if he is not able, then with his heart—and that is the weakest of faith."

Unless informing her guardian would lead to a greater harm, do not inform him. Instead, continue to advise her and remind her of Allah Almighty, threaten her, and pressure her by every possible means.