1) Welcome to the Party!
2) As you are obviously new here, I have some suggestions to help us all help you:
When you post, please post your entire configuration including (but not limited to) your installation method (7.X.X?) and vicidial version with build (VERSION: 2.X-XXXx ... BUILD: #####-####).
This IS a requirement for posting along with reading the stickies (at the top of each forum) and the manager's manual (available on EFLO.net, both free and paid versions)
You should also post: Asterisk version, telephony hardware (model number is helpful here), cluster information if you have one, and whether any other software is installed in the box. If your installation method is "manual/from scratch" you must post your operating system with version (and the .iso version from which you installed your original operating system) plus a link to the installation instructions you used. If your installation is "Hosted" list the site name of the host.
If this is a "Cloud" or "Virtual" server, please note the technology involved along with the version of that techology (ie: VMware Server Version 2.0.2). If it is not, merely stating the Motherboard model # and CPU would be helpful.
Similar to This:
Vicibox X.X from .iso | Vicidial X.X.X-XXX Build XXXXXX-XXXX | Asterisk X.X.X | Single Server | No Digium/Sangoma Hardware | No Extra Software After Installation | Intel DG35EC | Core2Quad Q6600
3)
... the telecommunication company of that 1234567 was configured to redirect the call to ...
The answer is in your question. Calls are never actually "forwarded" as a rule. A new call is generated which connects the old channel to the new channel (there are some notable exceptions, but the methodology in those exceptions function the same as if it were not an exception, so for today's answer ... doesn't matter! lol). When the forwarding/fresh call occurs, the "configuration" and decisionmaking process happens at the moment of forward. You at this end of the call have zero control over that decision. If the telco in question is configured to generate the new call with the original caller ID, they will. If they are configured to generate with some other caller ID, that's what will be seen. And there's absolutely nothing you can do from your end to manage this as it's not your equipment generating the new call.