Custom Extensions create two calls when dialling from FOP

We have created a new custom extension with exactly the same permissions and configuration as other working custom extensions.

If we dial in and then key in the extension, the call is diverted out to the mobile as normal.

However, if we then log into the FOP module as the extension and attempt to make a call the following happens:
Call is routed to mobile as normal
Call is picked up but then proceeds to voicemail everytime.
In FOP we see another incoming call against the extension in question
In GREP we see a duplicate call logged everytime.

We have also tried editing a pre-existing extension known to be working but get the same result whenever we change the outbound route to another number, though bizarrely it starts working again if we point it back at the original telephone number it diverts to