Do not use phone poles. They really are not very strong, especially at that span or even half that. No way in heck I'd drive a backhoe over a bridge built that way. For reference, I built a little bridge in the woods when I was about 14 using two large phone poles and it only spanned about 10'. Had to drive a small tractor over it once (IH 454, about 5000lbs) and it bowed a good bit under the load.
No matter what way you go you are looking at a lot of concrete and/or steel to do this properly and safely. I've built several bridges, fords, culverts etc for customers and a span that far is not something to mess around with. You are probably looking at four 40' I beams somewhere in the 12-18" range, and good solid concrete abutments on both shore, possibly supports in the middle as well. A project like that could go north of $10K real quick. The semi bed would be a good option if you can find one.
Now, that said, I'd go for either a ford or low water bridge. The ford is probably your best option and cheapest. You have a backhoe, so start on the side you can access and dig down to get a nice road down to water level. Bring in some good sized rock, 357 or larger and start lining your roadway with it and building your way across. If you widen out the stream there you can make it a shallower crossing. Depending on how deep it is it may take a good bit of rock, but it can be done. Make sure to keep a plenty wide base in the underwater section and try to make it as unobtrusive to the grade of the stream bottom as possible, otherwise it could blow out in a heavy flood. If your budget allows, you could pour a concrete wall along the lower side of the ford to help keep everything in place.
