Thanks to all of you who have been so generous with your help in resolving this issue. With the amazing help of two members of this forum, I now have a fully functional FreePBX implementation running on a Raspberry Pi, which has resulted in our achieving all of the aims we had set for ourselves with our VoIP set up. I cannot thank you all enough!
Gigasets voip->voip calls on the same base station are either unreliable or not working at all. We had plenty of issues with multiple Gigaset handsets on the same base station, such that we don't support this kind of set up with Gigasets on our phone system any longer.
Our advise is 1 base station - 1 handset.
Gigaset are a good phone with a great range, however the configuration is very illogical and limited and there are issues running multiple handsets.
We found that the new Grandstream cordless phones are easier to configure, you can run multiple handsets on the same base station with multiple extensions and call transfer facility. Unfortunately the range is not as good as on Gigasets, so they are more suitable for smaller offices.