Windows Phone 8 build in some hooks to make it easier for VOIP apps to integrate with the OS, but so far only Skype has really made use of it.
If however you use a generic SIP implementation the good news is that Acrobits are in the process of developing an app in Windows Phone that will allow access to most SIP servers.
This is according to Rafael Torreblanca, VP of Business Development from Acrobits S.R.O, who on enquiry said:
Thanks for your email. Yes we pare actually working on it, and we will release a version but only for end Q3 this year…
Their current iPhone client supports both SIP and Google Voice integration, which will hopefully carry over to their Windows Phone app also.
Read more about their apps at Acrobits here.
Thanks Blago for the tip.