TL;DR
A stranger approached me and asked me if they could use my phone. I didn't want to be rude and say no bluntly, so I wavered, and felt uncomfortable. How do I say no to strangers asking for use of my phone?
This is for future reference, this happened a few days ago and I would like to know what I (and others) can do in the future.
Background:
I was waiting for a bus at a bus stop, using my phone. A person approached me and asked me if they could use my phone to make a phone call. I wasn't sure what to do so I asked "to who?" and they answered "my brother" and I said "... to pick you up?" at this point I wasn't really sure of whether they were telling the truth or not. I finally ended by semi-confidentially saying that my bus was arriving soon, and I couldn't do that. After repeating a few times, they walked off.
In this situation, I couldn't just leave the location, and I was uncomfortable around a stranger I didn't know.
Question:
How do I tell a stranger no when they ask to use my phone for a personal phone call?
Reason I am asking on here is that I didn't feel confident when I said "no" that time, so I would like to have knowledge about what I can say and what is considered okay to say in this setting.
Notes:
I will not put down the appearance of this stranger, because I do not want any bias.
This was outside at a bus stop, near government buildings, downtown, in a relatively small city (population ~98,100).
This question is different from this one in that this is a setting where I did not indulge the stranger at all and did not want to lend the phone to the stranger at all. Additionally, I am in a more public place (on the street), not an enclosed place where social rules apply (the library).