Kyocera DuraXT / DuraPro

The absolute champ for reception of the recent-model Nextel phones is the i365, with "rubber-ducky" antenna. I have carried both the i580 and the i365 with rubber ducky antenna, and the latter wins hands-down in terms of reception. With the i365+rubber antenna, I have been able to make calls in areas where the coverage map shows no reception, and where the i580 gets no signal. Although it can't work miracles, in my experience the i365+rubber antenna is as good as you'll ever get for Nextel iDEN reception these days.

Sprint doesn't sell any new iDEN phones, but if you buy an i365 off ebay or wherever, they might switch over your existing i580 number to the i365. Call Sprint cust svc and ask if they can still swap numbers among existing Nextel phones.

Of course, Nextel & iDEN go away for good in another year, so perhaps switching to the i365 is not worth the effort for you at this time. If it is, this link shows how easy it is to get hold of and swap out the original pull-up antenna for a rubber ducky antenna on the i365:
https://forums.motorola.com/posts/c35c507023 »

Good luck whichever way you end up going.