Ian is correct IMHO. 
Let her calls roll over to voicemail, then you choose when to call her back. Choose a time that is convenient for you, but that gives you a reason to end the call in a timely manner (like when you’re driving and only 10 minutes form your destination). You can still be there for her (and keep her as a raving fan), but not let her make you crazy in the process. It works for me.