Is the following sentence gramatically correct? Thank you for your telephone call on April 12, 2008.
Yes, it is correct. However, idiomatically we tend to write "call of April 12, 2008."
And I've no idea why! We would certainly never say it - we would say it the way you did - or more likely, "Thanks for your call on April 12." It's just one example of the difference in written and spoken English.