Were the situations where they gave your their name and number such that it would be awkward for them to ask you for your number? Like the middle of a dance class or something like that?
Are you interested at all? If so, perhaps you could text them and have a "conversation" that way and if you are still interested give them your number and ask them to call you.
While i would think it would be better to strike up a conversation with someone I was interested in and then ask for her number, perhaps the situation wasn't conducive to doing that. Or perhaps they are a bit shy.
In all of these cases, it seems as though it would have been simple enough for them to initiate a conversation with me at the end of the event (dance class or lecture, in these particular cases).
I can't speak for them (being shy, in a rush to get somewhere else, etc.), as I don't know them.
My perspective is they had enough time to assess me and decide to write their info on a slip of paper to give me. Whereas I felt a bit blindsided by someone handing me something, muttering something, and quickly departing....as in "Who was that masked man?"
Maybe it's just me but I'd feel awkward in calling or texting someone..."Hello, you gave me your name and number at [event]...who are you?"
omni