If the therapist normally answers the phone herself then she shouldn't be surprised if you ask her who it is if she suddenly doesn't answer. That boundary is up to the T to keep- if she can't answer either let it go to voicemail. If she wants to use an answering service you should be made aware of this, either by her or the person answering the phone. Given the confidential nature of counseling random people shouldn't be answering a therapists phone line that they use specifically for therapy. If someone does her clients have a right to know who it is when they do call. Phone therapy should not done on one's private residential line.