Follow me Mystery

We have Samsung S3 running CSipSimple via office WiFi towards FreePBX hooked to our LAN. It works OK.

I am trying to get Follow Me to work like this … when the mobiles are out of office and not covered by our WiFi then I want that people calling into our office no and choosing via IVR an extension will go to that mobile via GSM network. It works OK with one exception - Busy

In Extension I have …

No Answer: Follow Me - (Force Follow Me)
Busy: Terminate Call - (Hangup)
Not Reachable: Follow Me - (Force Follow Me)

In Follow Me for that extension I have …

Follow-Me List: Sip Phone mobil no (with # at the end)

What happens is that when I am busy - like ext 101 talking to ext 102 - and get an external call (choosing ext 101 via IVR) that call will try to connect to ext 101 via its GSM network mobil no … but I have said it should terminate the call.
Have I got it all wrong ?