Hi,
I have gone through the requirement, I found that you are looking for a freelancer with Voip experience with Asterisk, A2billing,PBX, PHP, SIP for 1 month to work remotely.
I would like to suggest you for hourly base job cause If you are looking a developer for particular time period then their will be a hourly rate.
But Its all depends on your requirement, If we could make a conversation on chat to discuss in detail, Please let me know.
I am ready to sign NDA, If required.
Looking forward to do business with you.
Thanks,
Bhushan Nirmal