First, you need to decide on a number format for your system – users will dial calls in this format, incoming calls will display in this format, and any contact lists in the system or devices will use this format.
Unless you have special requirements, I recommend that you choose one of these:
Scheme A: 01 222 3333 (a number in Vienna); 00 33 1 2222 3333 (a number in Paris)
Scheme B: 43 1 222 3333 (a number in Vienna); 33 1 2222 3333 (a number in Paris)
Scheme A is usually better for businesses that are mostly domestic, because users dial and see numbers in the same format that their home and mobile phones use. If you do a lot of international business or have extensions in multiple countries, scheme B may be more suitable.
For outgoing, your Outbound Routes should recognize numbers in whatever scheme you choose and in most cases not modify them. For each Trunk, the Dialed Number Manipulation Rules should convert your internal format to whatever your provider requires. That way, a route can fail over to a provider that needs a different number format.
On incoming, any trunk that provides the caller ID in a different format should be routed to a context that rewrites the number to your internal format. If you have multiple trunking providers and they use different formats, you may need to create more than one such context.
If you still have trouble, post your desired internal format, what your trunking provider supplies for caller ID, present code for your context and what goes wrong; we can perhaps help you debug it.