Should I answer the phone?

My boyfriend is asleep and his mum is calling. She called twice but I wasn't sure if I should. If she calls again, do I answer? We've been together for 10 months. I'm comfortable with his mom, I see her often. My mom always told me to never be nosey and answer people's phone calls but what if something is wrong? I wouldn't care if he answered my phone while I was sleeping. please and thank you