Your VoIP requires that you reserve 640 Kbps of bandwidth full duplex. In view that its a T1 you do have control of both ends via the Samsung UBIgate 1000 and its QoS capability. The VoIP should be on a segregated subnet dedicated to VoIP. How do I arrive at 640Kbps? each VoIP phone will require 128Kbps full duplex and I assume that your office phones will all be active at the same time. Since your T1 provides 1.544 Mbps, full duplex and you need to reserve 640Kbps for VoIP that now leaves you with 600Kbps +/- full duplex available for data traffic. For typical text based mail [lacking graphic based attachments etc], surfing without streaming video the available bandwidth should be adequate. However if Video [YouTube, Graphics, etc..] is part of the mix then your data will crawl or freeze for the number of users you have.
IMO, you would be much better off with a good broadband ISP.--
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