Phone vs Email?

I sometimes just wonder, why do we always respond to phone call instantly but rarely do we reply an email right away?

i mean, both are instant communication tools and we make a phone call or send an email because we want instant feedback. but there's a lot of time that we don't usually respond to the email we receive right away, while it is more likely for us to receive a call or return a call faster.

so just a simple survey for you guys:

1. do you tend to return a phone call faster than an email? Why?

2. what keeps you from returning an email instantly? (afraid of letting people know you are always in front of the computer checking email? or consider email senders usually don't expect an instant feedback? or whatever reason?)

i am perhaps a little strange, coz i sometimes return email faster than a phone call, and i always try to return an email asap, but sure there are a lot of times that i have to reply to an email slower coz i notice that a lot of time an email response is usually more lengthy and require more effort to respond...