rockm

Telephone poles can be a real problem if you're not careful. They sometimes contain creosote (which can prevent rot, but can also cause some problem handling the things, not to mention, creosote is "aromatic" gives off fumes which your trees might not appreciate). If they have been used previously, they can also contain nails--which will destroy a chain saw and possibly you as the saw kicks back when you're cutting them down. Take note if you're doing this. Be careful.

Nayway, I believe telephone poles (and railroad ties) are simply too ungainly to work with.

The largest diameter square fence or deck post, 6x6" as Will said, work alot better. They're not as clunky, provide more tan adequate support and are easily workable. They have to be buried 24-30 inches in the ground--ust as most supports do-to be effective. You can make a simple flat top by laying out three or four deck rail slats with two parallel rail supports, then screwing the assembly to the top of the post. I've done thre or four of these and they support 100 lbs+ bonsai with no problem.

I have not cemented the posts into the ground though. It's not necessary either. After talking with professional fencing folks who installed my fence, I decided against it. They said the cement will actually wind up holding water against the wood inside the "cup" of cement that holds it. The posts rot much faster when cemented into the ground. Simply packing larger stones and backfilling and packing dirt into the hole allows drainage and support. This may just be the fencer's way of skimping, but I haven't had a post rot in years, while my nieghbors have replaced their cemented-in fence posts...