Snom M900 Multi Cell synchronisation problem with 3CX

Our customer has a site with 9 Snom M900 DECT configured in multicell setup with 3CX. All the DECT bases appear synchronized, however I have an issue with some M30 handset not registering when they are not in proximity of the primary base. Upon further investigation, I have noticed that out of 9 bases, only 3 have the 3CX extension on the them, despite all 9 of them appearing synchronized. I had to reset the 6 bases, and reactivate multicell setup one by one while waiting the synchronisation to finish. The issue got resolved but I didn't understand why the problem occurred and why it shows that all the bases are in sync, when some didn't have the 3cx accounts.

I appreciate if anyone had a similar issue while provisioning M900 bases with 3CX