dryaquaman

Could be the flash timers in the circuit descriptor of the analog line.
When you press the flash button on the phone, it is actually disconnecting the circuit just briefly and then reconnects. This signals the PBX the phone wants to do something. If the actual flash timer in the phone is longer than the flash the PBX is expecting the call will disconnect.
Find the Circ Desctiptor of the phone and play with the timers a bit.

Dry Aquaman

acejavelin

Does it fail when you press flash, or after you dial the extension, or when you hang up?

If the call disconnects when you press FLASH, check the timers like dryaquaman said, also see if the phone is actually doing a flash and not a disconnect. Some phones have a setting for the flash timer, make sure it is around 600ms to 800ms and not 1.5 or 2 seconds. Verify the settings in the ONS/OPS Circuit Descriptor are set correctly, normal settings in the US are Flash Type: Normal, Low Flash Timer 200ms, High Flash Timer 700ms. High Flash Timer is usually the culprit, try bumping it up to 850ms, but if you are going over 1000ms, then there is something not right about the phone.

If you can flash OK, then dial the extension and if it rings through and you can talk to the person you are in good shape. If you get a error tone, then check your Interconnect or tenant restrictions.

Now hang up, if the call gets disconnect or recalls back to you, verify that you have Broker's Call turned off in the sets Class of Service.

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