Calls Immediately Drop

[DCB]

Customer has Yealinks; A T31P and a W60B with a W56H. This is hosted in AWS and the SBC is running on Raspberry PI.
Randomly (more often than not) calls are dropped as soon as someone answers. That is to say, as soon as the handset is lifted off the T31 or the button pressed on the 56H; Doesn't even make it to the person's ear.
The caller can call back right away and the call connects just fine.

Apparently this has been going on for quite a while but recently it's gotten really bad.

I'm going on-site tomorrow morning. I'm hoping any late night folks will have a simple answer.

 

[SteveITS]

Does the 3CX firewall test pass?

There’s a call quality monitoring feature in v20 that might help.

 

[DCB]

It does. I'm onsite now. We are using Bandwidth.com for trunks. I ended up switching the IPs for the registrar server and outbound proxy. That seems to have fixed the issue.

If I create a realm in Bandwidth.com and use the realm name instead of IP addresses, will that work in 3CX?
Let me know if I should start a different thread for this question. I don't post on forums a lot so I'm not sure what proper etiquette is.