<PAGE>
 
                                                                   EXHIBIT 10.64
 
                              VERTEL CORPORATION
                             CONSULTING AGREEMENT
                             --------------------

This Consulting Agreement ("Agreement") is made and entered into as of the 5th
day of January, 1999 by and between Vertel Corporation and its successors (the
"Company"), and Philip Mantle ("Consultant").  The Company desires to retain
Consultant as an independent contractor to perform consulting services for the
Company and Consultant is willing to perform such services, on terms set forth
more fully below.  In consideration of the mutual promises contained herein, the
parties agree as follows:

     1.  SERVICES AND COMPENSATION
         -------------------------

          (a) Consultant agrees to be retained by the Company for consultation
services related to business strategy, market development and other areas of
knowledge and experience consistent with Consultant's prior work with the
Company (the "Services").  Consultant agrees to make himself reasonably
available to provide the Services as requested by the Company during the term of
this Agreement.

          (b) The Company agrees that Consultant shall be compensated for such
services through the grant and continued exercisability of a fully vested
nonstatutory stock option to purchase 62,969 shares of the Company's Common
Stock (the "Option").  The terms and conditions of the Option are set forth in
the Nonstatutory Stock Option Agreement attached hereto as Exhibit A.
                                                           --------- 

     2.  CONFIDENTIALITY
         ---------------

          (a) "Confidential Information" means any Company proprietary
information, technical data, trade secrets or know-how, including, but not
limited to research, product plans, products, services, customers, customer
lists, markets, software, developments, inventions, processes, formulas,
technology, designs, drawings, configuration information, marketing finances or
other business information disclosed by the Company either directly or
indirectly in writing, orally or by drawings or inspection of parts or
equipment.

          (b) Consultant will not, during or subsequent to the termination of
this Agreement, use the Company's Confidential Information for any purpose
whatsoever other than the performance of the Services on behalf of the Company
or disclose the Company's Confidential Information to any third party, and it is
understood that said Confidential Information shall remain the sole property of
the Company.  Consultant further agrees to take all reasonable precautions to
prevent any unauthorized disclosure of such Confidential Information including,
but not limited to, having each employee and/or subcontractor of Consultant, if
any, with access to any Confidential Information, execute a nondisclosure
agreement containing provisions in the Company's favor substantially similar to
Sections 2, 3 and 5 of this Agreement.  Confidential Information does not
include information which (i) is known to Consultant at the time of disclosure
to Consultant by the Company as evidenced by written records of Consultant, (ii)
has become publicly known and made generally available through no wrongful act
of 
<PAGE>
 
Consultant, or (iii) has been rightfully received by Consultant from a third
party who is authorized to make such disclosure.  Without the Company's prior
written approval, Consultant will not directly or indirectly disclose to anyone
the existence of the Agreement or the fact that Consultant has this arrangement
with the Company which is described in this Agreement.

          (c) Consultant agrees that Consultant will not, during the term of
this Agreement, improperly use or disclose any proprietary information or trade
secrets of any former or current employer or other person or entity with which
Consultant has an agreement or duty to keep in confidence information acquired
by Consultant in confidence, if any, and that Consultant will not bring onto the
premises of the Company any unpublished documents or proprietary information
belonging to such employer, person or entity unless previously consented to in
writing by such employer, person or entity.  Consultant will indemnify the
Company and hold it harmless from and against all claims, liabilities, damages
and expenses, including reasonable attorneys fees and costs of suit, arising out
of or in connection with any violation or claimed violation of a third party's
rights resulting in whole or in part from the Company's use of the work product
of Consultant under this Agreement.

          (d) Consultant recognizes that the Company has received and in the
future will receive from third parties their confidential or proprietary
information subject to a duty on the Company's part to maintain the
confidentiality of such information and to use it only for certain limited
purposes. Consultant agrees that Consultant owes the Company and such third
parties, during the term of this Agreement and thereafter, a duty to hold all
such confidential or proprietary information in the strictest confidence and not
to disclose it to any person, firm or corporation or to use it except as
necessary in carrying out the Services for the Company consistent with the
Company's agreement with such third party.

          (e) During the course of Consultant's services hereunder, the Company
will use reasonable efforts to minimize Consultant's exposure to sensitive
information that would cause Consultant to be restricted in his ability to trade
the Company's securities.

          (f) Upon the termination of this Agreement, or upon Company's earlier
request, Consultant will deliver to the Company all of the Company's property or
Confidential Information in tangible form that Consultant may have in
Consultant's possession or control.

     3.  OWNERSHIP
         ---------

          (a) Consultant agrees that all copyrightable material, notes, records,
drawings, designs, inventions, improvements, developments, discoveries and trade
secrets (collectively, "Inventions") conceived, made or discovered by
Consultant, solely or in collaboration with others, during the period of this
Agreement which relate in any manner to the business of the Company that
Consultant may be directed to undertake, investigate or experiment with, or
which Consultant may become associated with in work, investigation or
experimentation in the line of business of Company in performing the Services
hereunder, are the sole property of the Company.  In addition, any Inventions
which constitute copyrightable subject matter shall be considered "work made for
hire" as that term is defined in the United States Copyright Act. Consultant
further agrees to assign (or cause to be assigned) and does hereby assign to the
<PAGE>
 
Company all rights, title and interest in all such Inventions and any
copyrights, patents, mask work rights or other intellectual property rights
relating thereto.

          (b) Consultant agrees to assist Company, or its designee, at the
Company's expense, in every proper way to secure the Company's rights in the
Inventions and any copyrights, patents, mask work rights or other intellectual
property rights relating thereto in any and all countries, including the
disclosure to the Company of all pertinent information and data with respect
thereto, promptly after the development of such Inventions and the execution of
all applications, specifications, oaths, assignments and all other instruments
which the Company shall deem necessary in order for the Company to apply for and
obtain such rights and in order for Consultant to assign and convey to the
Company, its successors, assigns and nominees the sole and exclusive rights,
title and interest in and to such Inventions, and any copyrights, patents, mask
work rights or other intellectual property rights relating thereto.  Consultant
further agrees that Consultant's obligation to execute or cause to be executed,
when it is in Consultant's power to do so, any such instrument shall continue
after the termination of this Agreement.

          (c) Consultant agrees that if in the course of performing the
Services, Consultant incorporates into any Invention developed hereunder any
invention, improvement, development concept, discovery or other proprietary
information, the Company is hereby granted and shall have a nonexclusive,
royalty-free, perpetual, irrevocable, worldwide license to make, have made,
modify, use and sell such item as part of or in connection with such Invention.

          (d) Consultant agrees that if the Company is unable because of
Consultant's unavailability, dissolution, mental or physical incapacity, or for
any other reason, to secure Consultant's signature to apply for or to pursue any
application for any United States or foreign patents or mask work or copyright
registrations covering the Inventions assigned to the Company above, then
Consultant hereby irrevocably designates and appoints the Company and its duly
authorized officers and agents as Consultant's agent and attorneys in fact, to
act for and in Consultant's behalf and stead to execute and file any such
applications and to do all other lawfully permitted acts to further the
prosecution and issuance of patents, copyright and mask work registration
thereon or the securing or confirming of Company's fights hereunder with the
same legal force and effect as if executed by Consultant.

     4.  REPORTS
         -------

          Consultant agrees that he will from time to time during the term of
this Agreement or any extension thereof keep the Company advised as to
Consultant's progress in performing the Services, as set forth in Section 1, and
that Consultant will, as requested by the Company, prepare written reports with
respect thereto.  It is understood that the time required in the preparation of
such written reports shall be considered time devoted to the performance of
Consultant's Services.

     5.  CONFLICTING OBLIGATIONS.
         ------------------------

     Consultant certifies that Consultant has no outstanding agreement or
obligation that is conflict with any of the provisions of this Agreement, or
that would preclude Consultant 
<PAGE>
 
from complying with the provisions hereof, and further certifies that Consultant
will not enter into any such conflicting agreement during the term of this
Agreement.

     6.  TERM
         ----

          This Agreement will commence on the date first written above and will
continue until July 15, 2001.  This Agreement may be terminated earlier by the
Company only in the event that Consultant breaches the terms of this Agreement,
the terms of the Executive Separation Agreement entered into by and between the
Company and Consultant in January 1998 or the terms of the Stock Repurchase
Agreement entered into by and between the Company and Consultant dated November
27, 1998.

     7.  ASSIGNMENT
         ----------

          Neither this Agreement nor any right hereunder or interest herein may
be assigned or transferred by Consultant without the express written consent of
the Company.

     8.  INDEPENDENT CONTRACTOR
         ----------------------

          Nothing in this Agreement shall in any way be construed to constitute
Consultant as an agent, employee or representative of the Company, but
Consultant shall perform the Services hereunder as an independent contractor.
Consultant agrees to furnish (or reimburse the Company for) all tools and
materials necessary to accomplish this contract, and shall incur all expenses
associated with performance, excluding travel expenses (if any), which if
approved in advance by the Company will be reimbursed by the Company.
Consultant acknowledges the obligation to pay all self-employment and other
taxes thereon.  Consultant further agrees to indemnify the Company and hold it
harmless to the extent of any obligation imposed on Company (i) to pay any
withholding taxes or similar items or (ii) resulting from Consultant's being
determined not to be an independent contractor.

     9.  ARBITRATION AND EQUITABLE RELIEF
         --------------------------------

          (a) Except as provided in Section 9(b) below, the Company and
Consultant agree that any dispute or controversy arising out of or relating to
any interpretation, construction, performance or breach of this Agreement, shall
be settled by arbitration to be held in Los Angeles County, California, in
accordance with the rules then in effect of the American Arbitration
Association.  The decision of the arbitrator shall be final, conclusive and
binding on the parties to the arbitration.  Judgment may be entered on the
arbitrator's decision in any court of competent jurisdiction.  The Company and
Consultant shall each pay one-half of the costs and expenses of such
arbitration, and each shall separately pay its respective counsel fees and
expenses.

          (b) Consultant agrees that it would be impossible or inadequate to
measure and calculate the Company's damages from any breach of the covenants set
forth in Section 2 or 3 herein.  Accordingly, Consultant agrees that if
Consultant breaches Section 2 or 3, the Company will have available, in addition
to any other right or remedy available, the right to 
<PAGE>
 
obtain from any court of competent jurisdiction an injunction restraining such
breach or threatened breach and specific performance of any such provision.
Consultant further agrees that no bond or other security shall be required in
obtaining such equitable relief and Consultant hereby consents to the issuance
of such injunction and to the ordering of such specific performance.

     10.  GOVERNING LAW
          -------------

          This Agreement shall be governed by the laws of the State of
California.

     11.  ENTIRE AGREEMENT
          ----------------

          This Agreement, and the exhibit attached hereto, is the entire
agreement of the parties and supersedes any prior consulting agreements between
them.

     IN WITNESS WHEREOF, the parties hereto have executed this Agreement as of
the day and year first above written.

                                    CONSULTANT

 
                                    -------------------------------------
                                    By:  Philip Mantle

                                    Address:
                                    Street name
                                    City, Zipcode
                                    Country


                                    VERTEL CORPORATION

 
                                    --------------------------------------
                                    By:  Bruce Brown, President and CEO

                                    Address: 21300 Victory Blvd.
                                             Suite 1200
                                             Woodland Hills, CA 91367