As I have already explain nothing gets to the PBX system at all. We are using a variety of soft and hard phones with the PBX and only Zoiper on iPhone 6 has this problem. I am using Zoiper on Android and Windows and those are both working perfectly.
Also as already explained there is no error message of any description. Zoiper just disappears from the screen as soon as any number greater that 9 digits is dialled.
I have set IAX2 debugging on the system when doing this and no traffic is seen from the Zoiper soft phone at all on IAX2. If I dial a short number (such as a three digit extension number or a short feature code) then it works.