With SIP trunks, voice gets converted into IP packets. These IP packets then get sent to the SIP provider (and vice versa). Because voice is real-time sensitive, it requires that the voice packets arrive within a certain amount of time and in the right order (along with a number of other factors like jitter, packet loss, etc). If some of the voice packets get dropped or if they are sent too slow, audible quality issues can occur. So the quality of the connection between the customer’s phone system and the SIP provider directly affects the quality of the call. The connection between the customer’s phone system and the SIP provider varies. It can be a direct connection or over the Internet. The Internet is essentially a “network of networks” in which all types of data traffic (voice, video, email, web, etc) travel. Because the voice packets are “co-mingling” with other packets carrying various forms of information for different customers, it is considered a “best effort” form of delivery. This is why a dedicated connection between the customer’s network and the SIP provider is “technically” the best method for delivering SIP trunks. If that dedicated connection is also going to be used to send other types of data traffic (web surfing, email, etc), it needs to prioritize the voice packets over all other data packets (this is called Quality of Service).
That being said, a dedicated connection to the SIP provider is often too expensive for smaller customers. Delivering SIP trunks over a high bandwidth / low cost Internet connection is popular due to the enormous cost-savings. MOST quality issues are a result of a “bottleneck” or too much data on the” last mile” of the Internet connection. The last mile is the connection between the customer’s network and their Internet Service Provider (ISP). This can occur if the customer is trying to place SIP calls over the Internet while downloading large Internet files (for example). Bottleneck issues can be resolved by asking the Internet Service Provider (ISP) to provide quality of service (QOS – the prioritization of voice packets over all other traffic). Low-cost / High bandwidth providers will usually not provide this service. Another option would be to install a separate, dedicated Internet connection for the SIP trunks. This eliminates any possible bottleneck issues as long as the bandwidth can accommodate the number of simultaneous SIP calls needed. Last, it is always a good idea to use a Tier 1 or Tier 2 Internet provider to minimize the “hops” or networks the call must traverse to get to the SIP provider.
The best way to finish answering this question would be to give real-world examples of our customers with varying sizes, Internet traffic, and call volume:
Very small company with light phone traffic
Many of our smaller customers (usually 5 or less trunks) or branch offices have provisioned their SIP trunks over the same broadband Internet connection they use for standard Internet traffic. Because Internet and voice share the same “pipe” the amount of Internet traffic that customer generates and the size of the Internet pipe can affect the quality of the voice traffic. If there are quality issues in these circumstances, the cause is usually found in that “last mile” or the connection from the Internet Service Provider (ISP) to the customer. If the Internet and voice traffic is light, this can be a very good (and very cheap) solution for the customer.
Small to medium-sized company with moderate phone traffic
We generally recommend that the company who needs over 5 trunks with moderate phone traffic install a dedicated broadband Internet connection. Because most quality issues arise in the “last mile” or the connection between the customer and the Internet Service Provider (ISP), keeping standard Internet traffic (which is bandwidth-intensive) and voice traffic separate is a good idea. This eliminates any issues that arrive due to Internet traffic interfering with the voice traffic.
Medium to large company with moderate to large amounts of traffic
In these circumstances, we usually recommend that the company install a dedicated connection between their premise and the SIP provider (ITSP). Often, the SIP provider can provide this connection by bonding T1s for larger amounts of bandwidth. The SIP provider can also provide Internet access with Quality of Service for voice over this connection as well. Again, depending on the amount of voice and Internet traffic, the recommended connectivity solution and design will change. We have large customers who already have large Internet connection (DS3, OC3, etc) in which they’ve chosen to provision their SIP trunks over. Although these forms of connectivity are more expensive than your typical DSL or cable, the cost savings are still substantial after you consider long distance savings, the elimination of copper trunks at remote offices (with SIP, those numbers can port to the corporate office), better use of bandwidth (twice as many SIP trunks can be provisioned over the same amount of bandwidth as traditional PRI trunks), and other cost-savings factors.