Accuracy with GPS is multidimensional. In other words if you can locate yourself accurately to within 2 meters horizontally there is also a vertical +-.
When you throw in other technology (GPS assisted) I think the formula for determining acccuracy would need a supercomputer to run it. When using phone location first you would require a well designed/ well built municipal network. in other words the phone company has to have plenty of subscribers to pay for a solid infrastructure in which to use for geolocating suscribers (note: with the prescribed technology on the phone).
Then you have to have situations such as no overloading on the network bandwidth, interior location within the network, no overpowering magnetic events. I already mentioned that the technology has to match for the network and the phone so if you are traveling outside your subscribed area you may be on someone elses technology.
Then you have to consider that each carrier would inevitably implement the same type of technology with differing nuances, security, bandwidth, protocol hierarchy, etc.
All this being said it is still possible to measure the accuracy and provide a reasonable std dev formula that in most cases is going to fall within the +-.
Do some research of your own and find some known points on a map, go to them and take a reading (be sure to correlate the stated accuracy estimate to each point). spread about 5-8 points across your network (city coverage area). Then go outside the coverage area and take 2-3 readings.
I would imagine that outside the coverage area the carrieer would rely more heavily on gps, while determining location within the network is done by tower triangulation.
The answer is yes, as long as you take into consideration factors that can have a bearing on the accuracy and then weigh that against what the reading is.