Using "may" is asking permission - "Will it be o.k. if I call you?"
Using "can" is saying "Will I be able to call you?"
"May" allows them to say "yes".
If you use "can", how do they know if you will be able to call? You may be ill or your 'phone might be broken.
So, strictly you should always use "may".
However, in everyday usage, "may" and "can" tend to be regarded as the same.
Nobody will fail to understand what you mean whichever you use.
Hope this helps.