This may get a little more lengthy than most readers want.  But if RTK is of interest to you, it may help clear things up, or just add to the discussion. 

CORS: Continuously Operating Reference Station, an RTK base station that is always on. Data may be sent from the CORS via a radio and/or a network connections, such as the internet

VRS: Virtual Reference Station, is RTK. in most situations, VRS uses 6 CORS at a time, networked together correcting gps for greater RTK accuaracy and wide area coverage. If one of the stations is down, the next closest will be brought into the solution. VRS is a trademark of Trimble. Along the same lines, network RTK from Leica is Max and IMax. 

Tower Networks, as they are often refered, are not actually networked.  They are individual reference stations, albeit continuously operating, transmitting correction data via 900MHz to a receiver that is within range.  When moving around a wide area, selection of the nearest tower is needed.  Often times an operator needs to select a different tower while operating because the signal from one tower is lost or unavailable.

Accuracy:
This was taken from the Professional Surveyor Magazine, February 2003.
"Currently, VRS is predominantly being used by Mn/DOT, however, it's available free-of-cost to anyone." and "... need only a license, cellular modem, and the VRS software to gain the 1-2 cm horizontal accuracy that has been consistently gained with the new system."  The same article states "Three-centimeter vertical accuracy is common."

How you begin to measure this that precise in an agricultural environment is difficult. It is very accurate.

To achieve the same accuracy as a VRS system, most base station operators will say that the base station needs to be within about a mile.

Age of Data or lag time: how long of time it took the correction data to reach a receiver from the time it was created. Beacon DGPS it is usually 2-5 seconds, WAAS DGPS and other satelite corrections (Omnistar, SF1 and 2, etc.) 2-12 seconds, RTK needs to be 2 seconds or less. With the wrong connection or software this may be much greater than the needed 2 seconds or less.

For discussion sake, with the InTime connection on a EZ 500 or 332 running on the Minnesota VRS we see it at just over a second, while on the Wisconsin VRS we see it hold at 1 second. The Mojo will display in 1000ths of a second and while on the Minnnesota VRS we see it between 0.5 and 1 second. We have tested it through a Windows OS giving us 6 seconds (which is ancient for RTK) running on the Minnesota VRS, a 332 receiver and still maintaining 1 inch accuracy in a dynamic environment (moving tractor planting sweetcorn).

The advantage of using the cellular phone system is the indepth infrastucture that is in place.  It is robust with power and range.  It can handle alot of data, although the streamming of CMR+ correction data uses about 10% of the bandwidth that a voice call uses, and it is received only once per second.  Hardly a day goes by that we don't talk to someone about the limitations and shortcommings of their base station or tower network. Many users of these system are changing to the CORS or CORS/VRS system to greatly improve their RTK equipment. 

Other good reading

Trimble's VRS website just look at the number of white papers here and compare that to the white papers of all of agriculture GPS on the Trimble website.  Those doubting the ability of the VRS system, look at the middle section of page 7 of the RTK Network white paper.