Hello,
I am experience android developer and I can do this task for you, In past I worked with so many other open source like Linphone, Sipdroid, IMS droid, anti sip, CSipSimple etc. If you give me your SIP IP, username and password I will give you demo dialer with your ip address. I will give you best quality work with long time support, please have a look at my reviews and rating for quality of work I provide to client.
Thank you for taking time to read about my proposal, looking forward to hear from you.
Best Regards,
Silvans