VoIP doesn't have bandwidth, it uses bandwidth. To improve your bandwidth make sure you have an up to date modem and that you are subscribed to a large enough amount of data transfer from your ISP. Typically 1Mbps or higher is sufficient, but try for 3 Mbps minimally.
You may need to change your VoIP company, update the driver for your VoIP modem, or may need to purchase additional bandwidth through your Internet service provider. In some cases, you may need to actually replace your phone instead.
When making a VoIP call from your phone you will be more than likely accessing bandwidth from your carrier. If you are charged for your bandwidth usage or go over your bandwidth cap, you essentially will be paying fro the time you spend on your VoIP calls. Basically, yes.
QoS
VoIP
Cisco Express Forwarding and bandwidth of the interface
VOIP protocol utilizes Packet based streams for communication between VOIP endpoints. This makes the VOICE Communication prone to problems of Latency, Jitter and Packet Loss. Though QOS (Quality Of Service) features prioritizes the VOICE packets but still if VOIP is introduced with a VPN sort of Addressing mechanisms and efficient bandwidth reservation might ensure high performance for VOIP Technology.
I guess only the poor bandwidth of internet connection.
Use subnetting to break up the broadcast domain, as well as improve manageability. Also subnets will improve bandwidth usage.
The companies which offer the best VOIP bandwith plans are companies such as the following: Cisco, Facet Corporation, I Primus, and the Phone Booth website.
The reliability of VoIP is strongly dependent on your internet connection and service provider. It is important that your internet connection has at least 90 kbps of bandwidth, DSL is recommended in most cases. Overall VoIP is noted to be as reliable as the internet.