Hey, I am having more than 10 years of experience in VoIP and hands-on experiencing on Mobile dialer for Android and iOS, I am confident enough to work with you.
I have gone through your requirement, and you need a setup Linphone or similar open source VoIP.
Thanks for reading my proposal. Looking forward to hearing back from you soon.
(Note : Proposal amount may vary based on finalization of SOW)
Thanks & Regards,
Arpit