2) if you use some other computationally expensive hash you run into the problem of low powered mobile hardware. Remember you're not hashing 1 thing, but dozens or hundreds of phone numbers or email addresses.
You know, i think you're right - if we use different salts for each emails/phones, there'd be no way of ensuring that different users use the same salts, or even if we stored the salt along with the hash, it wouldn't be much use. Hmm this is a tricky problem!
2) if you use some other computationally expensive hash you run into the problem of low powered mobile hardware. Remember you're not hashing 1 thing, but dozens or hundreds of phone numbers or email addresses.