Call busy when answered

Dear,

I'm having some trouble with receiving calls on one of my DID's.

The situation goes as this:

2 phone numbers, each configured as dedicated trunk and both have a different inbound route connecting it to the same extension, which is a regular phone connected through an ATA.

When I call on number 1 it connects fine.
When I call on number 2 it sounds busy on the receiver's end, the caller just hears nothing and his connection isn't dropped.

When I use a software phone to receive calls on number 2 it works fine. It can't be the settings of the ATA, because it still works fine when calls from number 1 get connected.

Also, I've tried all the different RFC settings on both the trunk, extension and the ATA.

I've double checked everything. It doesn't sound logical at all.

Any ideas would be more than welcome.