There are lots of ways to use Cisco routers with advanced routing features that can do what you are looking to do. They all fall under "Quality of Service," or QoS for short. Cisco routers can regulate bandwidth in many many ways, such as marking, prioritization, queueing, bandwidth shaping, and traffic engineering.
It sounds like you want to make sure that voice traffic sounds great and gets prioritized over other types of IP traffic. This is one of the most common uses for QoS, and there are quite a number of documents and case studies on the Cisco website. You might want to look into the Cisco website for something called "low-latency queuing", or LLQ. This basically means that the traffic will be put into a queue that guarantees that it won't be waiting long to be sent out on the wire, no matter whether there is other traffic on the wire.
I found a good article that shows how to troubleshoot voice quality issues, and I especially like the way they explain how to mark traffic to be treated as priority, not only in your own network, but also further up the line. You can engineer your network to be the best for voice quality, but you also want upstream networks to honor that marking as well. Here's the link: http://www.cisco.com/c/en/us/support/docs/voice/voice-quality/20371-troubleshoot-qos-voice.html
Hope this helps!