In the last three weeks I have setup three new Symbian 3 phones.  An E7, C7, N8.  The E7 is parallel imported, the C7 came from 2 degrees, and the N8 unbranded from Nokia via a fundraising event.  All phones were updated to the latest software prior to be put in to use.  All phones are on the XT network.

Every phone within a a few days of being setup and put to use has lost the connection to the network. In other words show no network signal or the carrier (Telecom NZ).  The phone will work on WiFi but you are unable to make any calls, send or receive texts or use any data.  From what I can tell the phone isn't recognising the sim card.

I have resolved the issue by doing a soft reset using *#7780#.  So far this seems to have resolved the problem and the phones have worked normally from that time though for one phone the reset was only done today.

Whats causing this?  Is it a Telecom sim issue or a Symbiam Anna issue? Will it continue to be an ongoing problem?