A contract can be formed orally, for example through a telephone conversation. An oral agreement is binding in the same way as a written one, but it is more difficult to prove what has been agreed. 

If you are negotiating with a customer on the phone about an assignment, you should get their email address and then send a summary of the agreement via email. Like this, for example:

 “We just talked on the phone about a translation from Finnish to English. I’ve recapped the main points to be sure that we are definitely on the same page. The text is your engineering company’s annual report, which must reach the printing house by March 15. The text is ready to be translated by February 15.  Based on previous similar reports, the length is estimated to be 20.000 words, at 20 eurocents per word + VAT 24%, and it includes a proofreading round and one round of comments. The text should submitted for translation in rtf format and returned in the same format. If the length of the text to be translated differs significantly (more than +/- 10%) from the estimated length, the delivery time will be renegotiated before February 15, so that the printing schedule can be changed. Please get back to me as soon as possible if I have misunderstood something.”