A lot of consumer edge gear such as cable/adsl modems, consumer NAT routers, etc. have basic wireless access points built in. T-1 capable routers don't usually but there may be some. While these router/wireless combinations mostly work, it is very often the case that the logical place for the edge router (such as an electrically noisy data closet in the corner of the building) is far from the best place to provide decent wireless coverage.
For anything larger than an apartment-style residential setup, you're probably better off with a separate access point for your wireless. Because you have ten VoIP phones, you probably have a PoE switch that uses the Ethernet wiring to provide power. Many wireless access points (WAPs) are compatible with this which will simplify installation. A WAP can be configured on your data LAN and use the existing DHCP capability of your existing router.
Some WAPs can be configured with a second SSID and VLAN for guest use to separate casual users from your company network and just allow Internet access if you need this capability. With just a T-1 for internet and voice you probably don't want to offer guest use as you have limited bandwidth available.
Go with the IT consultant who is familiar with your particular environment rather than the ISP that doesn't know anything about it.
