Condo Call Box unable to complete call to my phone

I live in a gated building where we provide our phone numbers and they will give us back a 3-digit code. This code should allow guests to key in the 3-digit code from the gate's call box, and a call will be redirected to our phone, we then should press 9 to open the gate. 

When my code is used, the call box is unable to complete the call. It just goes straight to a busy tone. 

The manager at my complex has deleted and re-entered me into the system 3 times and that has not worked. They spoke with the gate company and they claim everything is fine on their end. They suggest that this could be a carrier issue. I troubleshooted with my carrier and they didn’t find any issues on my end. I had the manager input her own number to see if it was an issue with the call box, but when her number was the recipient for my code, the calls went through no issue.

I have an iPhone X (not sure if that matters here), but maybe there’s an issue with my phone? Has anyone else had this issue and got it resolved?