map,pop

my iphone is connected to email server through pop. my computer and other phone is also connected to email but through imap. The phone with the POP gets all the emails but when I delete one it is still on the sever whereas when I delete an email from the imap devices the email is also deleted from the server. When I try to change the POP device to IMAP it says "the POP server "map.cox.net" is not responding. Check your network connection and that you entered the correct information in the "incoming mail server" field. 

I would like all devices to be on the same page. When I delete from one they all update so that I'm not doing work twice. Under Advanced settings I have chosen "Delete from server when removed from inbox" selected but emails that i delete do not delete from the server. Any advice