Phone not ringing in call group

One of our customers has running a FreePBX version 2.8.1.5.
We have configured a ring group with 2 extensions.
When we call this group both extensions are ringing.
We are using ring-all strategy
The extensions are reachable.

The problem is when one extension is busy and we call the group the other (free) extension is not ringing.
Have anyone an idea what could be wrong?