U100 Answer Detection

Hello All,

So my company recently installed the Yealink U100 and things have been going well until today. One of my co-workers called into another conference call and after a few moments, the line hung up. When the call is placed, we get the connecting animation on the display of the phone. Normally this would then change to a timer to show that you are connected and how long the call has been going on. In this instance, the call is "never established" and so it hangs up. 

Can anyone point me in the direction to these settings and what would be the best configuration? What I have tried is going to the physical trunk and under the Analog trunk, the settings for the line for answer detection is 

Answer detection: Ring Detection

Custom Ring Tone: No

Everything else is grayed out.

More specs if you need:

- myPBX U100

- Analog line

- T42 phone

Thanks in advance

~ Alex Kearns