If you do not get a better answer (eg. by Cristian) and you have a service contract, think of opening
a service request and ask for configuration help.
Did you check on the options you find at the Settings ISDN page?
Could picture that the private flag is set untill you set "Specify national/international type of number".
Maybe its also related to Send "sending complete" or Legacy capabilities.
If you have a pbx admin available I would play around with these options and let him check
what might work. Anyhow, there also might be the way to configure the PBX to make it work.
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