Yes VG 204 is supported for SIP. I've not used it myself as SIP gateway though the procedure should remain same as of adding any other SIP Gateway ( i.e. add a SIP trunk & give the destination IP of the gateway ).
About ATA support with Ericsson, Ericsson ( or any partners with similar deployment experience ) should only be able to comment.
VG 2xx gateways can be used with both CUCM & CUCME.
About using them as individual device ( without any CUCM & CUCME ), I am not sure as I never used it like that but I did use a non-Cisco ( if I remember, it was called as MultiVoIP or something ) device which also provides multiple analog ports for connecting multiple analog phones & enable them for making outgoing calls over PSTN ( without using any Cisco PBX ). In this case, those analog phones will only be used for making outgoing calls, they wont expect receiveing incoming calls.
Integrating VG 2xx with CUCM/CUCME, will make those analog phones reachable by other IPT users & vice versa. This is apart from their PSTN calling ability as explained above.
I'm not able to access my old voice mail messages all of a sudden. The recording says something like 'the message is currently not available'. This has never happened before in all the years I have been using this system. I have t...