Before you register any DECT handset on the other sites, you first need to configure the DECT PARI of these sites to be the same like the DECT PARI of the Master site.
The DECT PARI can not be configured via the web-interface, this has to be done via provisioning.
Do not register any DECT handsets until the N870 multisite system has the same PARI as the master. Else you need to register all handset manually again.
On my second system the DECT PARI is:
On the second system you to the Provisioning page
"Browse" and select the xml file.
Click on Upload and Start auto configuration.
DECT PARI is now changed to the setting of the Master.
You can now register the DECT handsets.
How to Migrate an existing N870 multicell installation to an Virtual Integrator can be found here. FAQ Nx70 - How to migrate an N870 Multicell to an Virtual Integrator.