Changing caller id with a custom trunk

Ive set up my system and everything seems to be working fine apart from one small issue. When an incoming call comes in the system is removing the ‘0’ from the front of the number that we have here in the UK. The system is dropping the number from 11 digits to 10.
Ive researched the issue for days and have found many solutions but none of them will work with the custom trunk I have to use. Im using the sangoma A500 card and with this you need to use a custom woomera trunk. With this trunk there is no facility to change the ‘custom=’ feature within the trunk settings and it will not work just adding the extra information into the ‘extensions_custom.conf’ file.
I was wondering if anyone had any idea how to get around this problem. Any help will be greatly appreciated.

Many thanks