I´ve recently asked Microsoft Sweden which MS approved/certified SIP Trunk providers that can deliver as of today.
Interoute can deliver a SIP Trunk as of today. TeliaSonera are in the process of getting certified but are not there yet.
If you have more information of swedish SIP Trunk providers that supports Office Communications Server 2007 R2, please comment this article.
More information on SIP Trunking providers
Unified Communications Open Interoperability Program | TechNet
Dr Rez has written an excellent Technet Article on SIP Trunking!
SIP Trunking: What Is It? Why Do I Need It? How Do I Deploy It?