Though, there is no written etiquette regarding the number of times one should try calling or wait for certain time period during each subsequent call. However, there is a procedure I usually follow.
I try to call that person 2 times unless they pick the phone. I try calling them the second time as they might have been away from the phone and couldn't hear the ring. But before this, I wait for two minutes in case they heard the ring but couldn't pick the phone in time.
If it is the latter case, they will call me back. If it is former, then I call them again. In case they don't pick the phone this time, I will leave a text via any communication channel available like voicemail, FB, WhatsApp etc.
Leave a message something like,
I tried to call you without success. I was asking about [xyz]. Can you give me a call or text me back when you have the time?
Then I wait for an hour or so. If they respond to the text, then I've my answer. In case I don't, I wait for at least an hour and then give a call. At this point, I do not call them again for two-three hours as they might be too busy and can't pick the phone or ignoring me.
You should check the time you're calling them at. If they're at work, they could be busy. So try calling them during their break times if you know. If they are partying, they can't hear the ring due to noise. There can be other similar reasons. In your case she was in meeting but you didn't know so that's fine.
Whether they will call you back or not depends on where you are on their priority list. If you're in top priority, they will call you back soon and if you're not, you will have to call them back because they won't.
But in the end, patience is virtue.
From this article
Patience is a virtue. If you have a friend, family member, employee or co-worker who usually responds to your voicemails and texts, then there's probably a good reason why he/she doesn't answer. Before freaking out, panicking, getting mad, or sulking, consider the different possibilities. Consider all the times you had circumstances where you were not able to respond to others. If your friend or family member is tried and true considerate with returning calls and texts, trust that there is a good reason.
If you have a friend or family member who habitually doesn't respond to phone calls, voice mails, and sometimes texts, well then, there you go. They're not reliable.