Re:
Hi, i have the same problem. I have and Grandstream HT-488 on Caracas and i live in Belgium. My Grandstream 488 is an extension connected to a PSTN line, and he transfer my calls to my SIP Phone here in belgium, or if the SIP Phone is off, transfer the call to my cell phone using an Portech MV-370 with another extension number.
All the time i only see the extension number from the Grandstream (101), not the caller id from the PSTN incoming number in Caracas, because
3CX System only permit use the caller id from the postgres database.
I wrote this about a few months... http://www.3cx.com/forums/viewtopic.php?p=5170#5170
Maybe can be useful add this feature in a future release. My solution is add an line from another SIP provider in your 3cx system, and change the Grandstream for another number from the same provider, then the calling can be receive trought this external provider. But, some time i have problems with the 3CX system, because show PRIVATE as caller id and not the ID number. And i be sure is not and private caller id
So i think, you can receive in your cellphone the caller id from the original caller only if you cellphone provider permit change the originators callers ID, otherwise you only see you own phone number (the number was tranfer the call from SIP to GSM, in my case my own number in the MV-370).
Try with other provider and add this line to your 3cx system.