Call confirm not working in ring groups when using custom extensions

We have noted that when using custom extensions in ring groups the call confirm option fails with call er being told the call has been answered else where.

if the custom extension is replaced with a external number then the call works as expected with call confirm working. 

The custom extension does work as expected with a queue , so this seems to only affect ringgroups.

To test we have RG 8110 with a custom extenstion 8111 that calls Local/9XXXXX@from-internal we have also tried /n at the end as well