<PAGE>
                                                                  EXHIBIT 10.40

                            INSTRUMENT OF ASSUMPTION
                     AND SUBSTITUTION OF GUARANTOR OF LEASE


         This Instrument of Assumption and Substitution of Guarantor of Lease
("Assumption") is executed and delivered as of the 27th day of July, 2001 by
and between WORLD ACCESS, INC., a Delaware corporation ("Original Guarantor"),
VERSO TECHNOLOGIES, INC., a Minnesota corporation ("New Guarantor"), BOGGESS
HOLDINGS, L.L.C., a Utah limited liability company, as successor in interest to
BOGGESS-RIVERWOODS COMPANY, L.L.C., a Utah limited liability company
("Landlord"), and NACT TELECOMMUNICATIONS, INC., a Delaware corporation
("Tenant");

                              W I T N E S S E T H:

         WHEREAS, pursuant to that certain Lease Agreement dated December 30,
1999 (the "Lease"), Landlord's predecessor in interest Boggess-Riverwoods
Company, L.L.C. and Tenant entered into an agreement to lease an office building
located at 191 West 5200 North Street, Provo, Utah (the "Demised Premises");

         WHEREAS, pursuant to that certain Guaranty of Lease dated December 30,
1999 Original Guarantor guaranteed Tenant's obligations under the Lease; and

         WHEREAS, a wholly-owned subsidiary of Original Guarantor and New
Guarantor have entered into that certain Stock Purchase Agreement dated as of
June 4, 2001 in accordance with which New Guarantor has agreed to become
obligated on the Guaranty in substitution for Original Guarantor;

         NOW, THEREFORE, for and in consideration of these premises and the
terms and covenants contained herein, Original Guarantor, New Guarantor,
Landlord and Tenant hereby agree as follows:

         1. New Guarantor hereby assumes the Guaranty and all obligations of
Original Guarantor as "Guarantor" thereunder, including, without limitation,
assuming and agreeing to pay or perform in accordance with the terms thereof.

         2. Original Guarantor hereby grants, conveys, transfers and assigns to
New Guarantor any and all rights and obligations under the Guaranty.

         3. Landlord hereby covenants and agrees with Original Guarantor and New
Guarantor that the substitution of New Guarantor for Original Guarantor
hereunder shall be binding upon it and Landlord will look solely to New
Guarantor if and upon the event that Tenant defaults under the Lease. New
Guarantor shall be and is hereby substituted for Original Guarantor as
"Guarantor" in the Guaranty, and Original Guarantor is hereby released from any
and all obligations under the Guaranty.

         4. Paragraph 11 of the Guaranty is hereby revised in its entirety as
follows:

         All notices required, contemplated or made hereunder shall be in
         writing,


<PAGE>

         and shall be deemed to have been given properly and received upon the
         sooner of when (i) actually received or (ii) one day after deposit with
         Federal Express or another comparable overnight express delivery
         service, courier costs prepaid, addressed as follows:


                  If to Landlord:           Boggess Holdings, L.L.C.
                                            c/o Jerry R. Boggess
                                            671 South Ocean Boulevard
                                            Boca Raton, Florida 33432

                  With a copy to:   Parsons Behle & Latimer
                                            One Utah Center
                                            201   South Main Street, Suite 1800
                                            P.O. Box 45898
                                            Salt Lake City, Utah 84145-0898
                                            Attn:  Geoffrey W. Mangum, Esq.

                  If to Guarantor:  Verso Technologies, Inc.
                                            400 Galleria Parkway
                                            Suite 300
                                            Atlanta, Georgia  30339
                                            Attention:  Chief Financial Officer

                  With a copy to:   Rogers & Hardin LLP
                                            2700 International Tower
                                            229 Peachtree Street, N.E.
                                            Atlanta, Georgia  30303
                                            Attn:  Robert C. Hussle, Esq.

         or to such other address as may be specified by any party pursuant to
         written notification thereof, given by such party as required above at
         least ten (10) days prior to the effective date of the change.

         5. Landlord hereby consents to the sale of all of Tenant's stock and
grants its consent, to the extent such consent may be required or such
transaction may be deemed to be an assignment of the Lease. Landlord
acknowledges receipt of $1,500.00 in consideration for its reasonable time and
expense to review, approve and consent to this transaction and the substitution
of Guarantor.

         6. Landlord represents and warrants to New Guarantor that to the best
of its knowledge, as of the date hereof, there are no Events of Default (as such
term is defined in Section 21) under the Lease, and, to the best of Landlord's
knowledge, as of the date hereof, that no act or omission has occurred which
with the passage of time or the giving of notice, or both, would constitute an
Event of Default under the Lease.


                                       2
<PAGE>

         7. As an inducement to Landlord to execute this Assumption, Tenant
represents and warrants to Landlord that to the best of its knowledge, as of the
date hereof, Landlord is in compliance with its obligations under the Lease.

         8. Terms used herein and not otherwise defined shall have the meaning
set forth in the Guaranty, unless the context requires otherwise.

         9. This Assumption will be binding upon Original Guarantor, New
Guarantor, Landlord and Tenant, and their respective successors and assigns.

         10. This Assumption shall be governed by and construed in accordance
with the laws of the State of Utah.

         11. This Assumption may be executed simultaneously in counterparts,
each of which will be deemed an original, but all of which together will
constitute one and the same instrument. Executed counterparts may be delivered
via facsimile transmission.



                           [Signature page to follow]


                                       3
<PAGE>

         IN WITNESS WHEREOF, the parties hereto have caused this Assumption to
be executed by their duly authorized officers as of the date and year first
above written.



                                       ORIGINAL GUARANTOR:

                                       World Access, Inc.,
                                       a Delaware corporation

                                       By: /s/ W. Tod Chonar
                                           -------------------------------------
                                       Name:  W. Tod Chonar
                                             -----------------------------------
                                       Title: Executive Vice President &
                                              Secretary
                                              ----------------------------------


                                       NEW GUARANTOR:

                                       Verso Technologies, Inc.,
                                       A Minnesota corporation

                                       By: /s/ Juliet M. Reising
                                           -------------------------------------
                                       Name: Juliet M. Reising
                                             -----------------------------------
                                       Title: EVP & CFO
                                              ----------------------------------


                                       LANDLORD:

                                       Boggess HOLDINGS, L.L.C.,
                                       a Utah limited liability company

                                       By: /s/ Jerry E. Boggess
                                           -------------------------------------
                                       Name: Jerry E. Boggess
                                             -----------------------------------
                                       Title:
                                              ----------------------------------


                                       TENANT:

                                       NACT TELECOMMUNICATIONS, INC.,
                                       a Delaware corporation

                                       By: /s/ A. Lindsay Wallace
                                           -------------------------------------
                                       Name: A. Lindsay Wallace
                                             -----------------------------------
                                       Title: President
                                              ----------------------------------


                                       4