Customer have only 400 E1 DID blocks, they want to use the same directory number as IP Phone extension and FAX number, Please let me know it is possible.
You can share a DN between a fax (analog) port and an IP phone if the analog port is registered via SCCP.
However... you wouldn't want to. The fax, or the user wouldn't know if an incoming call was fax or a person.... who would answer?
Is there any way in the VG to detect the incoming call is for FAX [t.38] then forward the call to fax gateway or forward the call to CUCM.
Yes, you can do this but you shouldn't. It relies on a TCL script on the voice gateway to supervise the call and send it to the fax server when it detects a fax CNG tone. It's not perfect and some calls will slip through as voice. Depending how you configure it, it can also cause a penalty delay where the gateway is playing ringback while listening for a CNG tone before even sending the call to CUCM. In my opinion, the cost of additional DIDs is well worth avoiding this design. Plus, I believe this TCL script doesn't work with SIP trunking since it relies on DSPs to listen for that CNG tone. At least here in the US, ISDN circuits are falling off a cliff in favor of SIP trunks.
Here's the documentation but you've been warned.
I remember that thing (+5). I tried to forget...