If you live in a rural area with trees blocking line of sight to nearest Sprint tower and can't get a reliable connection, there is a solution.
My initial test was to put the Franklin in a bag taped to a long pole and raise the pole up to about 35 feet.Going from unreliable 1 bar to 2 bars, I knew that an external antenna would work.
The Franklin does have an external antenna connector.
Remove the back cover. Carefully remove the thin adhesive cover opposite the battery connector and opposite the reset button. (When looking at the display it will be on the lower left) The one labeled "Main" in the pictures in this thread:
I'm still not sure what connector type it actually is (TS9, MS-160, MS-162, ...) I used a RG316 MS162 Male Angle to FME MALE Coax RF Cable to connect to a 14 element Yagi antenna that I already had. I had to carefully remove the rim on in MS-162 angle connector with a Dremel tool.
On the debug screen I saw it was connecting on Band 26. Looking that up it's on 800MHz, so my 9 year old Yagi should work.
Mounted on a 50 foot pole (height is everything!), I went from unreliable intermittent 1 bar to reliable 2 bars and sometimes 3 bars. I would probably be getting better results with a brand new antenna and cable.