we have a 3CX system V18.0.237, we did a SIP trunk with a grandstream IPPBX, the trunk is up, is there any way that we route the call to the designated extension, not just in the extension mentioned in "Route calls to", like if I called from trunk side 2013 it should go to 2013 not to 2099.
That depends on how the calls are presented to 3CX when coming in and how your numbers are formatted and your source identification settings.
You should look at an Invite coming to 3CX and check where the called number is presented and match that setting in the inbound parameters of the PBX.
Since we do not support this functionality we have not tested it and we cannot provide assistance. You will need to do some troubleshooting on your own.
It sounds as if you are attempting something that would normally require the use of a Bridge trunk. That type of connection should allow direct dialing of an extension, where as "standard" SIP trunk won't.
In past 3CX versions, the use of Bridge trunks to connect to PBXs, other than 3CX. While not enthusiastically encouraged, it was, at least, supported, to an extent.
There was even a help page, which has since been removed...
However, that is no longer the case, and if you choose to attempt to connect with this method, while it may very well work for you, it would seem that it is now discouraged.
