<PAGE>

                                                                     Exhibit 5.1

July 14, 2000

American TeleSource International, Inc.

6000 Northwest Parkway, Suite 110
San Antonio, Texas  78249

     Re:  American TeleSource International, Inc. registration statement on Form
          S-3 (the "registration statement")

Ladies and Gentlemen:

     I have acted as counsel to American TeleSource International, Inc., a
Delaware corporation, ("ATSI") in connection with the registration for resale of
2,076,001 shares of common stock of ATSI (the "Shares") on Form S-3.    This
opinion is being furnished in accordance with the requirements of Item 16 of
Form S-3.

     I have reviewed the following:

          1.   ATSI's Amended and Restated Certificate of Incorporation (the
               "Certificate of Incorporation");
          2.   ATSI's Bylaws (the "Bylaws");
          3.   ATSI's Certificate of Designation, Preferences and Rights of 6%
               Series C Cumulative Convertible Preferred Stock (the "Series C
               Certificate of Designation");
          4.   Common Stock Purchase Warrant issued by ATSI to The Shaar Fund
               Ltd. dated September 24, 1999 (the "Series C Warrant");
          5.   Written Consent of the board of directors of ATSI dated September
               effective September 24, 1999 regarding the issuance of the Series
               C Preferred Stock;
          6.   ATSI's Certificate of Designation, Preferences and Rights of 6%
               Series D Cumulative Convertible Preferred Stock (the "Series D
               Certificate of Designation");
          7.   Common Stock Purchase Warrant issued by ATSI to The Shaar Fund
               Ltd. dated February 22, 2000 (the "Series D Warrant");
          8.   Written Consent of the board of directors of ATSI dated February
               15, 2000 regarding the issuance of the Series D Preferred Stock;
               and
          9.   Certificates of Good Standing for ATSI issued by the Secretary of
               State of the State of Delaware dated June 22, 1999 and February
               11, 2000.

               I note that all of the shares of common stock that were issuable
               under the terms of the Series C Certificate of Designation were
               issued on March 7, 2000, including
<PAGE>

          484,872 upon conversion of the Series C Preferred Stock, and 7,436 in
          payment of dividends due on the Series C Preferred Stock.

     Based on my review of these documents, it is my opinion that the Shares
issued pursuant to the Series C Certificate of Designation, when sold, will have
been duly authorized, validly issued, fully paid and nonassessable, and that the
Shares issuable under the Series D Certificate of Designation and the Series C
Warrant, and the Series D Warrant, if, as and when sold following issuance in
accordance with the terms of the Certificate of Incorporation and the Bylaws,
and in accordance with the terms of the Series D Certificate of Designation, and
the  Series D Warrant, or in payment of dividends on the 6% Series D Cumulative
Convertible Preferred Stock pursuant to the terms of the Series D Certificate of
Designation, as applicable, when sold, will be duly authorized, validly issued,
fully paid and nonassessable.

     I consent to the filing of this opinion as an Exhibit to the Registration
Statement and to the reference to me under the caption "Legal Matters" in the
prospectus which is part of the Registration Statement.  In giving this consent,
I do not admit that I come within the category of persons whose consent is
required under Section 7 of the Securities Act of 1933, as amended, or the Rules
and Regulations of the Securities and Exchange Commission thereunder.



                              Very truly yours,

                              /s/Alice L. King

                              Alice L. King
                              Corporate Counsel

                                       2