So sorry to hear about your friend's loss. You sound very considerate and kind.
Having just lost my baby at 20 weeks my experience (medically speaking) may be slightly different to your friend's so I'm not sure what she'll have just been through.
I expect she didn't answer the phone because she wasn't able too. I couldn't speak to anyone for days, especially not about the loss as I would just dissolve into tears if I tried to talk about it. I really appreciated receiving cards from people (and emails/texts, but the cards make nice keepsakes when you don't have many memories of your baby). It meant I felt like people cared, but I wasn't expected to respond. I would send a card, and follow up with the "thinking of you, no pressure but if you'd like me to pop over/phone just say" type texts/emails once or twice a week to let her know you're still available and haven't forgotten.
I really appreciate people talking about my baby (infact, as a big hint I have a photo of my baby's hands up in the living room). It's all I can think about at the moment and so I find small talk etc almost unbearable. Don't be afraid to mention her baby and her loss - you won't be reminding her of something she has forgotten about and upsetting her, she'll be thinking about it anyway. And please don't be uncomfortable if she cries - again, you haven't upset her and crying helps.
I find at the moment it takes me a long time to articulate my feelings so do leave big pauses if she's starting to talk about it all and let her have the space to work it through a bit. She will probably repeat herself a lot, explaining the same feelings, describing the same events several times. Just keep listening, she'll need time to process what has happened to her.
Refer to her baby rather than her embryo. I have to confess to having viscious thoughts about people who called my baby a fetus - it felt like they thought the loss was less significant then. (I am sure that's not what you meant, but I certainly am super sensitive!)
And as a tip, here are some top awful things to say - avoid these!
-"At least you can get pregnant"
-"You can try again/have another"
-"There must have been something wrong"
-"It's nature's way..."
Just tell her you are so sorry/don't know what to say/you are shocked/it's awful etc. Basically if you wouldn't say something to someone suffering a bereavement of a spouse/parent etc don't say it to her!
Good luck 