The Diagnostic code decimal 140 error is "Call with Incorrect Source Address", referring to the Network User ID or source address/DNIC. This means your calling gateway's address or address format is incorrect. The CUG number, if required, may also be incorrect.
Since you are attempting to log into another MacX25 server, that MacX25 server MUST be set up to receive calls! A MacX25 server by itself won't answer incoming calls.
* You'll need to be running the Apple Internet Router (AIR) AppleTalk/X.25 Wide Area Extension, or some other application on that server that makes MacX25 library calls to enable it to receive calls. If you are running the AIR X.25 Wide Area Extension, you'll need to set the port configuration to receive calls.
* You'll need to configure that receiving server's parameter file: open the parameter file, select Channel Allocation, select either two-way or incoming channels.
To imbed your local MacX25 server's X.25 address, launch ResEdit, open and edit the resource Xadd with your DNIC.
There are sometimes slight variations in addressee conventions used on private PDNs, such as you may need to precede the DNIC with an ASCII character, perhaps a 1 or 0. Since your calls on the public PDN work, but your calls on the point-to-point line do not, you'll want to inquire about any special calling/receiving conventions at some point required by that one PDN's "switch".