Starting from medium range routers like 2600, the Cisco routers are designed to handles speeds upto 2Mbps or higher. You can definitely go for Cisco based solution for Voice and data convergence after all Cisco is the leader in Routing Technologies.
If you want more info on Voice and Data integration using cisco products you can visit http://www.cisco.com/go/avvid to take a look at the industry proven AVVID solutions