Hello , The word "borrow" is for something mobile not static , so maybe for using a friend's land static phone at his own home you dont say can i borrow your phone , you rather say can i use it , as for cellulars there are mobiles and could be borrowed such as pens and lighters . Probably before the age of mobile phones no one used to borrow phones just used them ;)

Both ways are good. If you say 'could I borrow your phone' you may sound a little more polite.

If there is any mistake, it would be the use of could rather than may. Could expresses the ability to do something, while may expresses the permission to. However, these days people rarely use it correctly anyway.

Both are perfectly fine. When you ask the person if you could borrow it, he should understand that you're going to use it. Unless maybe when he knows that you like to use the Nokia 3310 as a throwing weapon.

I would say its perfectly fine to say both ... Who told you that??