While the issue is happening, can you send a ping to the SIP Server's Windows Server 2012?

Also can you access to outside web site from the web browser running under the same Windows at that time?

When did the issue start happening? Did you change any settings just before the issue started happening? How about memory and CPU usages when the issue happens? Are they high? Are you using a VM product such as VMware for running Brekeke SIP Server? Which Telcom product are you using with Brekeke ...

Do you have DialPlan rules? please make sure they don't make an infinite loop.
Have you rebooted the SIP Server or OS after the issue happened? and did it solve the issue?

Since you are using Brekeke PBX, you don't have to edit DialPlan.

For accepting a call sent from the SIP trunk, PlacetelPBX , add a new ARS.

http://wiki.brekeke.com/wiki/ARS
http://wiki.brekeke.com/wiki/Setup-for-Connecting-ITSP