cold call

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A telephone call or visit made to someone who is not known or not expecting contact, often in order to sell something.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A sales call either by telephone or personal presence, made without a referral or without preparing the recipient of the call.
• verb To call someone without preparation or referral.