Just want an appointment?! what gives?!

So far during my pregnancy I've had two appointments, a dating ultrasound at 6 weeks, and then I had my regular first trimester pregnancy appointment at 9 weeks. When I was there, my doctor told me to go see her receptionist to take an appointment for my next ultrasound/appointment at 20 weeks. This was in November. So I go to the receptionist and she says she "isn't sure about something" and to just call her at the beginning of December to make it. I told her I work (and with children no less) for the last 2 hours of her shift, so if anything, if I don't pick up, that I would call RIGHT back as my boss is understanding. So I did call at the start of December, on the 2nd. I called. And I called. And I called. Voicemail every single time. For days! FINALLY I got a call back when I was at work at 3:40 (honestly... And they close at 4) so I literally called back within ONE minute and it went right back to voicemail. I leave yet another message. The next day, she does the same exact thing, but this time at like 3:56. So obviously when I call back, she doesn't pick up. Since then, I've been calling for weeks and leaving voicemails and can't get a phone call back. It's ridiculous. I've called the main hospital number who just redirected me to her. And the ultrasound department who also sent me to voicemail. What should I do now? I know 20 weeks is still a little bit away for me but my doctor wanted me to get an appointment ASAP because she's super busy.