I have got some additional information on this issue:
For the IPECS version 5.0 (E.0 Dj),
you need to check for the settings for PGM133 and PMG126.
PGM133

This corresponds to the PGM350 of the ipLDK-100/300)
*1. Set SIP Attributes in PGM 133 as shown below.
1) Soft Switch Type = Normal (or Broadsoft).
2) Proxy Server Address : x.x.x.x (public IP address).
3) Primary DNS Address : "DNS#1 IP Address"
4) Secondary DNS Address : "DNS#2 IP Address"
5) Use outbound proxy -> ON.
6) Connection Mode = UDP/ON.
7) Set Domain Address as "x.x.x.x"
(More value can be set accordingly, but these are the most important)
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PGM126 (for ALL SIP users, this must be done):
(This corresponds to the PGM 351 of the ipLDK-100/300).
PGM126:
1. First set the Index number one-by-one in the top fields.
First Index is # 1.
Then set Order # of PGM126 as below.
#1) User ID : 100@x.x.x.x
#4) Contact number : 1000(Station number - e.g. 1000)
#7) User ID Registration : Register
#9) User ID Usage : True / ON
2. Set SIP User ID Table number of Station 1000 as 01.
This gives Station number 100 the SIP Index #1 (01).
PGM + 111 + 10001000 + BTN56+ 01 + [SAVE]
3. Reset IPECS manually.
4. Check if the user is registered to SIP Server normally.
5. Call the DDI number xxxx1000, registered in PGM231, as usual.
Makes sense?
IP address of x.x.x.x is your Public IP address.
Let me know about your progress.
///doktor