Why do my calls to my mom go straight to voicemail?

In the last month, at some point, when I would call my mom, it would ring once and then get cut off by going straight to voicemail, my mom did not block me or has had her phone turned off, its being doing this for awhile now, every time I try to call her, she has to call me, so I text her to call me, and I get the notification that I am getting a call and I answer, so I can still talk on the phone with her and text her but I literally cannot call her from my phone and I don't know why and looking my problem up on the internet has not been helpful, its just annoying, I have tried deleting her contact and just calling her number and I have also restarted my phone serveral times and checked my settings to see if it was set to send to voicemail, I just dont know what else to do.

[Re-Titled by Moderator]