When a call comes in, or a call is made outbound on SIP and is, for example, placed on hold, the call cuts off.
From what I have seen, this is down to a codec problem in the configuration.
Connect to the system using Manager E and navigate to the following location:
Settings | Auxiliary Equipment | Announcement
Once here, you will need to check the codecs in the bottom right hand corner under:
MOH Sources | Audio Codecs
Check which codecs are configured and make sure that the top 2 are ones supported by your SIP carrier, e.g. in the UK, set the top 2 to G.729a and G.711a
As far as I know, this can only be done through Manager E and not through the Web Based Management, as this option does not seem to appear. This may change in later releases and this How To will be updated if and when it does.