Phone coaching is one of the essential elements of Dialectical Behavior Therapy (DBT), a very effective form of psychotherapy for BPD.
In DBT, you will be provided with both group skills training (a therapy group in which you will learn basic skills for managing emotions, maintaining relationships, and tolerating distress), and individual psychotherapy with a therapist. In addition to meeting with your therapist in person, individual DBT therapists usually provide phone coaching. In fact, unlike other forms of therapy for BPD, in DBT you are encouraged to call your therapist.
Phone coaching is used when you are in a crisis and in need of some help. The goals of phone contact are to decrease suicide threats and attempts, to help you to apply skills you've learned in DBT to your everyday life, and to help you maintain a healthy relationship with your therapist.
Most DBT therapists do provide some guidelines about the use of phone coaching. For example, if you are calling too frequently or using telephone calls inappropriately (e.g., calling but being unwilling to accept the help your therapist provides), your therapist will likely work with you to change how you are using the phone coaching.
Linehan, MM. "Cognitive Behavioral Treatment of Borderline Personality Disorder." New York: Guilford Press, 1993.