NEC UNIVERGE SV9100 Features And Specifications page 1160

the country where the primary system is located. SV9100 NetLink connections are only
supported within the same countries/areas. For example, the SV9100 can be connected via
NetLink between the US and Canada, however the SV9100 can not be connected via
NetLink with systems in other countries (e.g., Mexico or the U.K.).
Synchronous Ringing via NetLink is not supported.
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