Hello Matthew
I think you would go for very cheap setup.That is on ATA box.this technology can be only ok for 10 seats.It has some drawback.But can be workout in 10 seat.if you would go to this technolgoy less price.But if you need to increase seats this setup would not used for expansion.Big setup has diffrent things.
If you dont to go with this.You can go by rent on basis in india.
If not.Than better work go with VOIP technology.You need any help with process also.Please provide your contact details.we can help you out.
Regards
Lalit Tyagi