I think you are misunderstanding this... there are two ways things use SIP in the Mitel 5000 (and most other systems), once is line-side SIP, where you use SIP devices like the UC Softphone or a Polycom SoundPoint IP phone and they use SIP to connect to the 5000 instead of MiNET or other protocol, in this case the 5000 is essentially the SIP provider and no external SIP connections are needed to the public network provided you have the correct licensing. The other SIP service is trunk-side, where you get SIP trunks from a SIP provider, such as Clearfly or Broadband.com, to provide inbound and outbound calling to the public world, you do not need this in order to make SIP devices work.