One of my sales agency friends installed a new high speed internet/telephone system about 4 years ago from a company called cBeyond. It operated over a T-1 line (a business class high speed internet connection). Plugged into that T-1 line was a telephone system called a PBX (private branch exchange—once the providence of only very large companies).
Feature phones were plugged into the PBX. Total cost was in the $10,000 range for a 15 extension installation with 5 or 6 telephone lines connected to a single telephone number (usually called roll-over lines). The telephones had lots of features like caller ID, call forwarding, voice mail, the ability to transfer calls to another extension, music on hold, etc.
The telephone calls went to the telephone central exchange via the T-1 data line. Any data capacity left over was available for high speed internet to the business. Coupled with fractional T-1 offerings from the phone company you can specify as much capacity as you want (and can afford).
Long distance calls back then were a couple of cents per minute; I’m guessing they are included now. And call quality was as good, and many times better, than regular telephone calls.
There are also virtual VoIP options that don’t even require a PBX, but give the same functionality (http://www.virtualpbx.com/index.asp was the first service that Google presented). One advantage is you can get more features (like call directors, call queuing, etc) and they support your employees taking their calls from anywhere. JetBlue is famous for having its reservations work from home, now any small business can have the same capabilities.
Staying with traditional telephone service, or depending on your cell phone for long distance, may not be the best business decision. At least investigate VoIP, especially for your field sales people.