Did you leave the message with an actual nurse? I think that entirely changes the situation. I would not worry about not hearing anything back until the end of the day then, they are trained to triage calls.
My questions go to nurse voice mail and they check it a few times a day and I generally get a phone call in a few hours. Then I explain it all to the nurse again and most of the time I'm put on hold and she talks to the OB or can answer it on her own. It seems like they set aside time right then to handle it together that way than playing tag. I've overheard what sounded like that in the office as well. And it's a group practice.
If it's just about a test and just about how it fits in your schedule, I would give them a free pass that it's pretty low on their list of important things to handle, especially given the nature of some OB practices where the doctor is out on deliveries whenever. That happened even in my group practice last time, they usually have 2 doctors at the hospital/day (split shifts) but every once in awhile they need another one! You don't know the kind of day they were having and obviously it wasn't urgent. As a person who's done scheduling for an office, if you need the test then you will find time to schedule it. Sorry if they couldn't perfectly accommodate you.
This doesn't pertain to you at all, but nothing would irritate me more than a parent calling in claiming their child had to be seen today and I only had x time- "oh, well that won't work at all? Don't you have z time?" "No, not today. I have that tomorrow." "Oh, ok, tomorrow then." Um, you just said how important it was to be seen but now it's fine if doesn't suit your schedule.
Oh, and personally, I'd get used to that. It's a pain in the last few weeks when you have to schedule weekly appointments and you have to see x doctor (because they all take turns) but he/she is only in on this day and they only have this time available. It gets to a point where you just take what they give you and say thank you. Really not looking forward to that this time when dealing with either taking a toddler or arranging care for him.