Voice quality and busy lines can be addressed by using different solutions. The best and most affordable is to use VoIP with at least 24 Kbps/agent, but some solutions also offer lower bandwidth consuming products like 16 Kbps and 8 Kbps. Since the bandwidth costs in Pakistan are extremely high, for the best quality I would recommend sticking to 24 Kbps/agent when planning your costs. You can put about 30 agents on a T1, costing you about USD 90/agent/month.
If you go for an IP Phone system (like Vonage or Superphone or Outbox), it consumes about 30 Kbps and will cost you around USD 100/agent/month for bandwidth.
If you use VoIP and redundant bandwidth, you can provide uptimes of 98-99.5%. If you need more assistance I’d be glad to help.