I have a customer ordering a SIP trunk from a service provier and replacing their e1 link. they have been told they can add a sip trunk directly from CM with out a gateway - is this correct they don;t need CUBE or anything?
ok so best to configure CUBE and terminate the SIP trunk on the gateway then SIP trunk from CM to the gateway?
then NAT external address to internal address of CUBE gateway?
Typically the CUBE does it's own NAT. See this document: http://www.cisco.com/en/US/docs/ios/12_4t/ip_addr/configuration/guide/htnatsbc.html
I take it you dont have to plug the SIP trunk directly into the gateway only needs to be a logical connection?
For security purposes, most customers/providers utilize a private Ethernet connection between the SIP provider and CUBE. If you don't have this option, you will need to add a firewall to protect CUBE in the DMZ. Regardless of the deployment, use SIP digest authentication, registration, and an ACL to restrict SIP communications with the router.
also is their much config required?
Not really but a fair amount of reading upfront to understand what you are doing. http://www.cisco.com/en/US/docs/ios/voice/cube/configuration/guide/vb_book/vb_book.html