Hi,
I am an IT engineer with computer and electrical engineering as major in my academic background.
I am also a VoIP expert and an Asterisk specialist with 7 years of experience.
Your task requires low-level development skills(Linux driver development). My opinion is that, budget can't be less than $500 (may be starting from $750 at least).
I can do the job, and I have the lab (Raspberry Pi model B and Huawei usb dongle).