Ahsan,
I worked in Maryland as well in Bethesda. Currently I am in Islambad. I have helped setup one international call center and now deploying my second one with 60 seats.
1- Superphone’s voice quality is very good. But it uses around 30Kbps of bandwidth just for voice. Plus it is all manual dialing and no CRM/DB integration at all. With 12 seats I would not recommend just using Cisco ATA 186 i.e superphone. Recommended are dedicated gateways with FXS/FXO ports. They are availble in 2,4,8,16 and 24 ports
2- Just for voice with a suitable compression algorithm; theoretical requirements are 16Kbps for a call BUT I would allocate at least 24Kbps
I hope this info helps; let me know if u need anything else.
Regards
Saeed Qadri