So the problem occurs only when dialing from PBX2 to Cisco via PBX1, correct?
Question back to you: if this is IP Trunking via STMI (on 4K), why are the calls from PBX2 to Cisco tandeming thru PBX1 instead of being routed directly to the IP Address of the Cisco Gateway?
In PBX2 you have LDPLN dial pattern entries 1XX, 2XX, 3XX, 4XX routed to PBX1. In this route's LDAT config, the Out-Dial Rule should be "ECHO 1","END", since the LDPLN pattern contains only 1 field.
Also in PBX2, you have LDPLN pattern 7XXX. Based on your previous post, did you break up this pattern into "7-XXX", or combine it as "7XXX"? If the dial pattern is configured in LDPLN as "7-XXX", and the Cisco system needs to see all 4 digits, then your Out-Dial Rule for this pattern would be "ECHO 1","ECHO 2","END". But if the LDPLN pattern is "7XXX", the Out-Dial Rule should be "ECHO 1","END";
But the Out-Dial Rule may not be the problem. I suspect that since you added the Cisco dialing scheme yourself, you may have missed one critical trunking parameter that is usually relevant only in TANDEM scenarios: LCOSV. When a call from PBX2 sends the digits to PBX1 which are intended to be routed to the Cisco system, PBX1 must re-process the call, as if dialed by a "user" in PBX1. In a trunk tandem scenario, this "user" is the trunk itself. Therefore the "LCOSV" assigned to the PBX1 trunk circuit must be capable of placing the call to the Cisco system. Since you said that users/subscribers in PBX1 can dial the Cisco switch, DISPLAY one PBX1 station that CAN successfully place a call to the Cisco system, and note its LCOSV1 and LCOSV2 values (1 = DAY time, 2 = NIGHT time). Now DIS-TDCSU for the IP Trunk in PBX1, and verify that the trunk circuit uses the same LCOSV value as the station's LCOSV1 value. If not, then perform CHA-TDCSU and change parameter "LCOSV". Then re-test your call.
It could also be an LCR Authorization problem, but I am betting on LCOSV. Both the LCR Authorization and LCOSV are configured in AMO COSSU.
Good Luck. I AM NOT supposed to be HERE.