SIP client in Ubuntu Touch?
Solution 1:
The package source for Ubuntu Touch's system apps can be found here:
Touch/CoreApps - Ubuntu
This includes the default app, Dialer. As it stands, however, VoIP and SIP don't appear to be integrated with the Ubuntu Touch OS. As such, you would need external APIs to do the Dialer integration you mentioned.
There are two other pages that support this assertion:
Google Drive - Sources
It's really quite unfortunate, seeing that this would have given Ubuntu Touch a more competitive edge in the mobile market :(