I bought another phone on ebay, as so many of my friends have been telling me to do this. It was from a seller who guaranteed that it was a sprint phone, and who had high marks from past customers. Ordered it late on Thursday and received it on Monday, complete with the battery, wall and car charger, for only $29.95 total, shipping and all.
As long as it is a Sprint phone yes. I tried to activate the palm treo 755 online but had to call Sprint as I kept getting error messages. They had to remove the phone from the account it had been registered to and then they were able to give me directions over the phone to activate it to my account. Took only a few minutes once I called them. Good luck!
What you want to do is commonly called an "ESN swap." What you're doing is simply replacing one phone for another on the account without making plan changes. If the phones are identical models, this is a no-brainer. You can even do it from within your account on sprint.com.
If the phones are different models, you might run into some grief, as Sprint will only activate certain phones on certain plans. This is particular true of "smartphones" where Sprint will for some but not all models, ask you to pay an additional $10/month, though their explanation of exactly why this is is infuriatingly vague and seemingly irrational. (It's required for 4G phones, but it's not a fee to use 4G, for example.)
If the phones are largely similar in capabilities and no plan change is required (which should be true for basic phones), the ESN swap is easy, but you have to make sure the phone you're adding has a "clean" electronic serial number or ESN. If the previous owner didn't pay their bill or the phone was stolen, its ESN will be added to a "hotlist" or "blacklist." If it's on one of those, Sprint won't activate your phone.
To do an ESN swap, log into sprint.com and click on "My Sprint" in the tabs at the top of the page. Look down to your list of devices, and pull down the menu next to the one you want to swap out, selecting "Activate a new device." You'll be asked to enter the ESN information for the new phone.
I hope this helps.
Check out a fun infographic that teaches you how to increase storage, enhance security, improve battery life and clean your device.