The phone GPS can be used to save a location from within the Field Entry editor. This is presented in eastings/northings but is stored as longitudel/latitude since that is the native format of the builtin GPS.
You can change the coordinate system on the fly via the location block dropdown menu, or change it for all items via settings.
All phones are different; it is important you check the GPS accuracy of your device and make sure that it is appropriate for your needs. Phone GPSs are not the most reliable; they are designed for gaining a rapid fix ahead of gaining an accurate fix. It is possible to buy devices that replace phone GPSs and operate via bluetooth. Unless such a device it attached, the phone GPS location should be assessed carefully before using it.
The quoted accuracy of the GPS reading is based on a 1 standard deviation error. A stated accuracy of 3m means that 67% of the time, the reading will actually be within 3m, or 95% of the time it will be within 9m etc.
In order to achieve the best results with a phone GPS it is suggested that you leave the phone in one location undisturbed while it gains a fix. If the accuracy is low, then immediately press the refresh button to search for a more accurate reading.