First of all:
Your network connectivity between the SIP provider and your system needs to be setup in such a way that the INVITE messages from your SIP provider arrive at your system.
If you have a "static IP with no authentication" SIP trunk then this should be happening without any REGISTER messages from your system.
It's possible that by sending the REGISTER message to SIP provider you are enabling for the return connection from SIP provider to you to work, where as it would not work otherwise.
Right now you have VoiceGuide system set up to send out a REGISTER message on service restart.
If after a few minutes pause in communications you no longer see INVITE messages from VoIP provider then, then it looks like you do not have static IP routing in place.
If after a few minutes pause in communications you no longer see INVITE messages from VoIP provider then, then it looks like the initial sending of REGISTER message enabled the routing through the NATs in between your system and the SIP provider, and these routes timed out after a few minutes.
You need to speak to people who are responsible setting up network connectivity. Network routing needs to be set so that messages from VoIP service provider always arrive at your system.
Try removing the "VoIP_Registration" entry in VoiceGuide's Config.xml and restating the VoiceGuide service.
Most likely you will then not see any VoIP calls arriving at your system - as static IP routing has not been set up.
If you cannot get your network administrators to set up this static IP routing, then you can try workarounds - which can involve repeated sending of REGISTER messages from your system to the SIP provider. This repeated sending of REGISTER messages can refresh NAT tables. If the static routing has not been set up then this constant refreshing of NAT tables may be necessary to allow communications.
Does this mean I can ask for username and password even for Static IP environment?
It's not usually needed in 'static IP routing' setup, so it would be an unusual request to make. You can see if your SIP provider will give you this option, if they do then you can use the Register(+Authenticate is needed) as a workaround until the static route has been set up properly.
The 're-sending or REGISTERs' is done if you have setup the "VoIP_Registration" entry in VoiceGuide's Config.xml, and the SIP service provider is confirming the registrations are successful.
Usually for registration to be successful you need to Authenticate as well - and to do that you need to setup the "VoIP_Authentication" entry in VoiceGuide's Config.xml as well.
Right now your SIP service provider is sending a '401-Unauthorized' in response to your REGISTER message, so you need to ask your SIP provider for user/password which you can use to Authenticate (or ask them to not require user/password, and just accept REGISTER based on your IP address, but most would not allow this)