CISCO Undocumented Commands

x25 version 1980

Indeed the IOS clears the call because the host is encoding a modified
destination address without signaling the reason in a CLAMN facility
encoding.

According to the X.25 standard, section 6.26 ("Called Line Address
Modified notification"), a DTE may set the the CLAM facility in a Call
Return packet to indicate that the called address has changed. If this
facility is not set, the call may be cleared. This is what you see
happening.

2 options could help us :

1)
Earlier IOS releases and x25 IOS part did not verify the CLAM
facility. Since IOS 11.3T, however, the X.25 code has been rewritten
and this check has been introduced for standards compliance.

We can configure the hidden commmnd "x25 version 1980" on all X.25
interfaces that the call is passing (serial 1 for you).

It can help but might introduce other problems.
Let's give it a try.

eventually provide me with a new debug and a fresh "sh run"

2)
If the switch which is directly connected to the router doesn't care about
the X25 called adress to route the X25 VC, i think you can use under serial1
"x25 suppress-called-address"

So, by using the suppress-called-address command under the x.25 serial
interface, the router will not check the address in the call-confirm.