<PAGE>

                                                                   EXHIBIT 10.20


                             STOCK OPTION AGREEMENT

     THIS STOCK OPTION AGREEMENT (this "Agreement") is made as of December 20,
                                        ---------
1999 (the "Effective Date"), between IWO Holdings, Inc., a Delaware corporation
           --------------
("Holdings"), and Solon Kandel (the "Optionee").
  --------                           --------

                                    RECITALS
                                    --------

     WHEREAS, Optionee is an employee of Holdings, and Holdings desires to grant
the Optionee the opportunity to acquire a proprietary interest in Holdings to
encourage the Optionee's contribution to the success and progress of Holdings.

     WHEREAS, the Board of Directors has as of the Effective Date granted to the
Optionee a non-qualified option to purchase shares of Class B Common Stock,
$0.01 par value, of Holdings (the "Class B Common Stock") subject to the terms
                                   --------------------
and conditions of this Agreement.

     WHEREAS, this Agreement has been approved by the Board of Directors and the
stockholders of Holdings.

                                   AGREEMENTS
                                   ----------

     1.  Definitions.  Capitalized terms used herein shall have the following
         -----------
meanings:

     "Act" is defined in Section 9(a).
      ---

     "Affiliate" means (a) any Person which, directly or indirectly, is in
      ---------
control of, is controlled by, or is under common control with, such Person or
(b) any Person who is a director or officer (i) of such Person, (ii) of any
subsidiary of such Person or (iii) of any Person described in clause (a) above.
For purposes of this definition, "control" of a Person means the power, directly
or indirectly, (x) to vote 50% or more of the securities having ordinary voting
power for the election of directors of such Person whether by ownership of
securities, contract, proxy or otherwise, or (y) to direct or cause the
direction of the management and policies of such Person whether by ownership of
securities, contract, proxy or otherwise.

     "Agreement" means this Stock Option Agreement.
      ---------

     "Approved Sale" means a transaction or a series of related transactions
      -------------
which results in a change of economic beneficial ownership of Holdings or its
business of greater than 50% (disregarding for this purpose any disparate voting
rights attributable to the outstanding stock of Holdings), whether pursuant to
the sale of the stock of Holdings, the sale of all or substantially all of the
assets of Holdings, or a merger or consolidation; provided that a sale of stock
by an Initial Stockholder to (i) another Initial Stockholder or Affiliate
thereof, or (ii) a non-U.S. entity with respect to which an Initial Stockholder
or Affiliate thereof has an administrative relationship shall be disregarded
when determining if an Approved Sale has occurred.
<PAGE>

     "Certificate of Incorporation" means the Amended and Restated Certificate
      ----------------------------
of Incorporation of Holdings, as amended from time to time.

     "Cause" has the meaning set forth in the Employment Agreement.
      -----

     "Class B Common Stock" is defined in the Recitals.
      --------------------

     "Closing Date" means December 20, 1999.
      ------------

     "Daily Vesting" means vesting of Time Vesting Options proportionately for
      -------------
each of the 1,095 days on or after January 1, 2000 that the Optionee is employed
by Holdings such that if Optionee remains employed for all 1,095 days such Time
Vesting Options shall be vested in full.  Daily vesting will commence on January
1, 2000.

     "Disability" has the meaning set forth in the Employment Agreement.
      ----------

     "Discounted Vested Option" is defined in Section 3.
      ------------------------

     "Effective Date" is defined in the preamble.
      --------------

     "Employment Agreement" means the Employment Agreement between Optionee and
      --------------------
Holdings, dated as of the date hereof, as amended from time to time.

     "Exchange Act" means the Securities and Exchange Act of 1934, as amended.
      ------------

     "Exercise Price" is defined in Section 2.
      --------------

     "Fair Market Value" means (a) prior to an Initial Public Offering:  the
      -----------------
value of Holdings' shares determined in good faith by the Holdings' Board of
Directors.  The Board of Directors shall make its determination of Fair Market
Value annually (the "Annual Valuation") promptly after the completion of
                     -----------------
Holdings' audited financial statements for the year then completed and such
determination shall remain in effect until the Board of Directors makes the next
Annual Valuation.  Notwithstanding the foregoing, if the Board of Directors or
an investment banker or appraiser appointed by Holdings makes a determination of
Fair Market Value subsequent to an Annual Valuation, such subsequent
determination shall supersede the Annual Valuation then in effect and shall
establish the Fair Market Value until the next Annual Valuation.  The Fair
Market Value shall be based on an assumed sale of 100% of the outstanding
capital stock of Holdings (without reduction for minority interest or lack of
liquidity of the Option Shares or similar discount).  If such determination of
the Fair Market Value is challenged by the Optionee, the Board of Directors will
select an appraiser or investment bank with a national reputation who is
independent (not an Affiliate of the parties) (the "Appraiser") and who shall
                                                    ---------
establish the Fair Market Value as of the date of valuation referenced in the
Annual Valuation or a subsequent determination.  The Appraiser's determination
shall be conclusive and binding on Holdings and Optionee.  Holdings shall bear
all costs incurred in connection with the services of such Appraiser; and (b)
subsequent to an Initial Public Offering:  The price of the last sale of the
stock on the date Optionee exercises the Option.

                                       2
<PAGE>

     "Fiscal Year" means the fiscal year of Holdings.
      -----------

     "Initial Public Offering" means the sale of any of the common stock of
      -----------------------
Holdings pursuant to a registration statement that has been declared effective
under the Act, if as a result of such sale (i) the issuer becomes a reporting
company under Section 12(b) or 12(g) of the Securities Exchange Act of 1934, as
amended, and (ii) such stock is traded on the New York Stock Exchange or the
American Stock Exchange, or is quoted on the NASDAQ National Market System or is
traded or quoted on any other national stock exchange or national securities
system.

     "Initial Stockholders" means the stockholders of Holdings who became
      --------------------
stockholders as of the Closing Date (including employees or directors of
Holdings or any Subsidiary who were granted options to purchase stock as of the
Closing Date) and any transferees of such stockholders described in clause (i)
or (ii) in the definition of Approved Sale.

     "Option" is defined in Section 2.
      ------

     "Optionee" is defined in the preamble.
      --------

     "Option Shares" is defined in Section 2.
      -------------

     "Person" means an individual, partnership, joint venture, limited liability
      ------
company, corporation, trust, unincorporated organization or a government or any
department or agency thereof.

     "Retirement" means age 65.
      ----------

     "Subsidiary" means any joint venture, corporation, partnership or other
      ----------
entity as to which Holdings, whether directly or indirectly, has more than 50%
of the (i) voting rights or (ii) rights to capital or profits.

     "Termination Date" means the date on which the Optionee ceases to be
      ----------------
employed by Holdings for any reason.

     "Vested Option" is defined in Section 3.
      -------------

     2.  Grant of Option.  Holdings grants to the Optionee the right and option
         ---------------
(the "Option") to purchase, on the terms and conditions hereinafter set forth,
      ------
all or any part of the number of shares of Class B Common Stock set forth below
the Optionee's signature below (the "Option Shares").
                                     -------------

     3.  Exercisability.
         --------------

     (a) The Option shall be divided into three parts for purposes of
determining exercisability.

                                       3
<PAGE>

     (i) The Option to purchase 11,235.9551 shares of Class B Common Stock (the
"Discounted Vested Option") at a purchase price of $43.521 per Option Share
 ------------------------
shall vest on the Effective Date.

               (ii)   The Option to purchase 16,853.9326 shares of Class B
          Common Stock (the "Vested Option") at a purchase price of $172.34043
                             -------------
          per Option Share shall vest on the Effective Date.

               (iii)  The Option to purchase 28,089.8876 shares of Class B
          Common Stock (the Time Vesting Option") at a purchase price of
                            -------------------
          $172.34043 per Option Share shall vest on the basis of Daily Vesting,
          commencing on January 1, 2000 and continuing through December 31,
          2002.

          (b) Notwithstanding Section 3(a), (i) upon the occurrence of an
Approved Sale, in which case the schedule set forth in Section 3(a) shall not
apply with respect to Time Vesting Options that are not yet exercisable, 100% of
the Time Vesting Options shall become exercisable immediately prior to the
closing of the Approved Sale and (ii) if the Optionee is terminated without
Cause or employment is terminated due to death or disability, 100% of the Time
Vesting Options shall become exercisable immediately prior to the Termination
Date.

     4.  Expiration.
         ----------

         (a) Subject to Section 6(a), the exercisable portion of the Option
shall expire on April 30, 2009, unless:  (i) the Optionee is terminated by
Holdings with Cause or voluntarily leaves prior to the expiration of the
employment term pursuant to the Employment Agreement, in which case the
exercisable portion of the Option will expire on the later of (A) three years
after the Termination Date or (B) two years after an Initial Public Offering, or
(ii) the Optionee is terminated without cause; the Optionee's employment is
terminated due to death, Disability or Retirement; the Optionee voluntarily
leaves and the Board of Directors determines that the Optionee left Holdings due
to personal hardship; or the Optionee voluntarily leaves following the
expiration of the initial employment term pursuant to the Employment Agreement,
in which case the exercisable portion of the Option will expire on the later of
(A) five years after the Termination Date or (B) two years after an Initial
Public Offering.

         (b) The unexercisable portion of the Time Vesting Option shall expire
on the earlier to occur of (i) except to the extent provided in Section 3(b),
the Termination Date or (ii) except to the extent provided in Section 3(b) and
Section 6(a), an Approved Sale, .

     5.  Nontransferability.  The Option shall not be transferable by the
         ------------------
Optionee other than by will or the laws of descent and distribution except that
the Optionee may transfer the Option to (a) his or her spouse, child, estate,
personal representative, heir or successor (b) a trust for the benefit of the
Optionee or his or her spouse, child or heir, or (c) a partnership or limited
liability company the partners or members of which consist solely of the
Optionee and/or his or her spouse, children or heirs (each, a "permitted
                                                                --------
transferee") and the Option is exercisable, during the Optionee's lifetime, only
----------
by him or her or his or her spouse or child, or, in the event of the Optionee's
Disability, his or her guardian or legal representative.  More particularly (but
without

                                       4
<PAGE>

limiting the generality of the foregoing), the Option may not be assigned,
transferred (except as aforesaid), pledged or hypothecated in any way (whether
by operation of law or otherwise), and shall not be subject to execution,
attachment or similar process.  This Agreement shall be binding on and
enforceable against any person who is a permitted transferee of the Option
pursuant to the first sentence of this Section.

     6.  Effect of Approved Sale; Adjustments.
         ------------------------------------

         (a) In the event of an Approved Sale, the unexercised portion of the
Option shall terminate upon such Approved Sale unless (i) provision is made in
writing in connection with such Approved Sale for the assumption of such
Options, or for the substitutions of such Options of new awards covering the
securities of a successor entity or an Affiliate thereof, with appropriate
adjustments as to the number and kind of securities and exercise prices, in
which event such outstanding Options shall continue or be replaced, as the case
may be, in the manner and under the terms so provided; or (ii) the Board of
Directors otherwise shall provide in writing for such adjustments as it deems
appropriate in the terms and conditions of the then-outstanding Options,
including without limitation (A) accelerating the vesting of outstanding Options
and/or (B) providing for the cancellation of Options and their automatic
conversion into the right to receive the securities, cash or other consideration
that a holder of the shares underlying such Options would have been entitled to
receive upon consummation of such Approved Sale had such shares been issued and
outstanding immediately prior to the closing date of the Approved Sale (net of
the appropriate option exercise prices).  If pursuant to this Section 6(a) the
Options are to terminate upon an Approved Sale without provision for any of the
actions described in clause (i) or (ii) above, then the Optionee shall be given
at least ten (10) days' prior notice of the proposed Approved Sale and shall be
entitled to exercise such exercisable but unexercised portion of the Option
(including all options that become exercisable immediately prior to the Approved
Sale pursuant to Section 3(b)) at any time during such ten (10) day period up to
and until the close of business on the day immediately preceding the date of
consummation of such Approved Sale and upon exercise of the Option the Option
Shares shall be treated in the same manner as the shares of any other holder of
Class B Common Stock.

         (b) Subject to Section 6(a), if the shares of the Class B Common
Stock, or to the extent it affects the economic rights of the holders of the
Class B Common Stock, shares of Class A Common Stock, Class C Common Stock or
Class D Common Stock of Holdings, are changed into or exchanged for a different
number or kind of shares or securities, as the result of any one or more
reorganizations, recapitalizations, mergers, acquisitions, stock splits, reverse
stock splits, stock dividends or similar events, an appropriate adjustment shall
be made in the number and kind of shares or other securities subject to the
Option and the price for each share or other unit of any securities subject to
this Agreement.  Any such adjustment in outstanding Options shall be made
without changing the aggregate exercise price applicable to the unexercised
portions of such Options.  No fractional interests shall be issued on account of
any such adjustment unless the Board of Directors specifically determines to the
contrary; provided, however, that in lieu of fractional interests, the Optionee,
upon the exercise of the Option in whole or part, shall receive cash in an
amount equal to the amount by which the fair market value

                                       5
<PAGE>

(as determined in good faith by the Board of Directors) of such fractional
interests exceeds the Exercise Price attributable to such fractional interests.

     7.  Exercise of the Option.  Prior to the expiration thereof, the Optionee
         ----------------------
may exercise the exercisable portion of the Option from time to time in whole or
in part.  Upon electing to exercise the Option, the Optionee shall deliver to
the Secretary of Holdings a written and signed notice of such election setting
forth the number of Option Shares the Optionee has elected to purchase and shall
at the time of delivery of such notice tender cash or a cashier's or certified
bank check to the order of Holdings for the full Exercise Price of such Option
Shares and any amount required pursuant to Section 16 hereof.  Alternatively, if
Holdings is not at the time prohibited from purchasing or acquiring shares of
its capital stock by applicable law or under the terms of any debt or lease
facility, the Exercise Price may, at the option of the Optionee, be paid in
whole or in part by delivery of shares of the Class B Common Stock owned by the
Optionee provided that Optionee has owned such shares for at least six (6)
months.  The value of any such shares delivered or withheld as payment of the
Exercise Price shall be such shares' Fair Market Value.  In addition, Holdings
shall cooperate with Optionee to facilitate a sale of Option Shares through a
broker to pay the exercise price provided such sale is otherwise permitted under
this Agreement, the Certificate of Incorporation or the Stockholders Agreement
or under applicable law.  The Board of Directors further may, in its discretion,
permit payment of the Exercise Price in such other form or in such other manner
as may be permissible under any applicable law.

     8.  Stockholders Agreement.
         ----------------------

          The Option Shares are subject to the terms and provisions of the
Stockholders Agreement (the "Stockholders Agreement") by and between Holdings
                             ----------------------
and the Stockholders, as such term is defined in the Stockholder Agreement, and
the Optionee shall be treated as a Class B Stockholder under the Stockholders
Agreement with respect to the Option Shares.  Section 6 of the Stockholders
Agreement shall not apply to any surrender of Option Shares to Holdings in
connection with the Optionee's exercise of this Option as contemplated by
Sections 7 and 14 hereof.

     9.  Compliance with Legal Requirements.
         ----------------------------------

         (a) No Option Shares shall be issued or transferred pursuant to this
Agreement unless and until all legal requirements applicable to such issuance or
transfer have, in the opinion of counsel to Holdings, been satisfied.  Such
requirements may include, but are not limited to, registering or qualifying such
Option Shares under any state or federal law, satisfying any applicable law
relating to the transfer of unregistered securities or demonstrating the
availability of an exemption from applicable laws, placing a legend on the
Option Shares to the effect that they were issued in reliance upon an exemption
from registration under the Securities Act of 1933, as amended (the "Act"), and
                                                                     ---
may not be transferred other than in reliance upon Rule 144 or Rule 701
promulgated under the Act, if available, or upon another exemption from the Act,
or obtaining the consent or approval of any governmental regulatory body.

                                       6
<PAGE>

         (b) The Optionee understands that Holdings intends for the offering
and sale of Option Shares to be effected in reliance upon Rule 701 or another
available exemption from registration under the Act and intends to file a Form
701 as appropriate, and that Holdings is under no obligation to register for
resale the Option Shares issued upon exercise of the Option, subject to other
applicable agreements or the Certificate of Incorporation.  In connection with
any such issuance or transfer, the person acquiring the Option Shares shall, if
requested by Holdings, provide information and assurances satisfactory to
counsel to Holdings with respect to such matters as Holdings reasonably may deem
desirable to assure compliance with all applicable legal requirements.  Holdings
shall use its best efforts to register the exercise of the Option under a
registration statement on Form S-8 within a reasonable time following the
closing of an Initial Public Offering.  Holdings shall take reasonable steps to
cause this Agreement and the exercise of the Option granted hereunder to comply
with the exemption from Section 16 of the Exchange Act provided under Securities
and Exchange Commission Rule 16b-3 or any successor rule, as it may be amended
from time to time.

     10.  Subject to Certificate of Incorporation.  The Optionee acknowledges
          ---------------------------------------
that the Option Shares are subject to the terms of the Certificate of
Incorporation.

     11.  No Interest in Shares Subject to Option.  Neither the Optionee
          ---------------------------------------
(individually or as a member of a group) nor any beneficiary or other person
claiming under or through the Optionee shall have any right, title, interest, or
privilege in or to any shares of stock subject to this Agreement except as to
such Option Shares, if any, as shall have been issued to such person upon
exercise of an Option or any part thereof.

     12.  Not an Employment Contract.  Nothing in this Agreement or any other
          --------------------------
instrument executed pursuant thereto shall confer upon the Optionee any right to
continue in the employ of Holdings or any Subsidiary or shall affect the right
of Holdings or any Subsidiary to terminate the employment of the Optionee with
or without Cause.

     13.  Governing Law.  All terms of and rights under this Agreement shall be
          -------------
governed by and construed in accordance with the internal laws of the State of
New York, without giving effect to principles of conflicts of law.

     14.  Taxes.  The Board of Directors may, in its discretion, make such
          -----
provisions and take such steps as it may deem necessary or appropriate for the
withholding of all federal, state, local and other taxes required by law to be
withheld with respect to the issuance or exercise of the Option including, but
not limited to, deducting the amount of any such withholding taxes from any
other amount then or thereafter payable to the Optionee, requiring the Optionee
to pay to Holdings the amount required to be withheld or to execute such
documents as the Board of Directors deems necessary or desirable to enable it to
satisfy its withholding obligations, provided further that the Optionee may
satisfy all aforesaid withholding tax obligations by directing Holdings to
withhold that number of Shares with an aggregate Fair Market Value equal to the
amount of all federal, state, local and other taxes required to be withheld, or
delivering to Holdings such number of previously held shares of capital stock of
Holdings which shares have been owned by the Optionee for at least six (6)
months with an aggregate Fair Market Value equal to the minimum statutory amount
of the federal, state, local and other taxes required to be withheld.

                                       7
<PAGE>

     15.  Transfer Notice.  Holdings will provide the Optionee with the Transfer
          ---------------
Notice (as defined in the Certificate of Incorporation) delivered to Class B
Stockholders pursuant to Section 4 or Section 5 of the Certificate of
Incorporation in accordance with the terms thereof.

     16.  Notices.  All notices, requests, demands and other communications
          -------
pursuant to this Agreement shall be in writing and shall be deemed to have been
duly given if personally delivered, telexed or telecopied to, or, if mailed,
when received by, the other party at the following addresses (or at such other
address as shall be given in writing by either party to the other):

     If to Holdings to:

          IWO Holdings, Inc..
          c/o Gibson, Dunn & Crutcher LLP
          200 Park Avenue, 47th Floor
          New York, New York 10166
          Attention:  E. Michael Greaney, Esq.

          With a copy to:

          Gibson, Dunn & Crutcher LLP
          200 Park Avenue, 47th Floor
          New York, New York 10166-0193
          Attention:  E. Michael Greaney, Esq.

     If to the Optionee to the address set forth below the Optionee's signature
below.

     17.  Amendments and Waivers.  This Agreement may be amended, and any
          ----------------------
provision hereof may be waived, only by a writing signed by the party to be
charged.

     18.  Entire Agreement.  This Agreement, together with the Stockholders
          ----------------
Agreement, sets forth the entire agreement and understanding between the parties
as to the subject matter hereof and supersedes all prior oral and written and
all contemporaneous oral discussions, agreements and understandings of any kind
or nature.

     19.  Separability.  In the event that any provision of this Agreement is
          ------------
declared to be illegal, invalid or otherwise unenforceable by a court of
competent jurisdiction, such provision shall be reformed, if possible, to the
extent necessary to render it legal, valid and enforceable, or otherwise
deleted, and the remainder of this Agreement shall not be affected except to the
extent necessary to reform or delete such illegal, invalid or unenforceable
provision.

                                       8
<PAGE>

     20.  Headings.  The headings preceding the text of the sections hereof are
          --------
inserted solely for convenience of reference, and shall not constitute a part of
this Agreement, nor shall they affect its meaning, construction or effect.

     21.  Counterparts.  This Agreement may be executed in two counterparts,
          ------------
each of which shall be deemed an original, but which together shall constitute
one and the same instrument.

     22.  Further Assurances.  Each party shall cooperate and take such action
          ------------------
as may be reasonably requested by another party in order to carry out the
provisions and purposes of this Agreement.

     23.  Remedies.  In the event of a breach by any party to this Agreement of
          --------
its obligations under this Agreement, any party injured by such breach, in
addition to being entitled to exercise all rights granted by law, including
recovery of damages, shall be entitled to specific performance of its rights
under this Agreement.  The parties agree that the provisions of this Agreement
shall be specifically enforceable, it being agreed by the parties that the
remedy at law, including monetary damages, for breach of any such provision will
be inadequate compensation for any loss and that any defense in any action for
specific performance that a remedy at law would be adequate is hereby waived.

     24.  Binding Effect.  This Agreement shall inure to the benefit of and be
          --------------
binding upon the parties hereto and their respective permitted successors and
assigns.

                                       9
<PAGE>

     IN WITNESS WHEREOF, the parties have executed this Agreement as of the
Effective Date.
                              IWO HOLDINGS, INC.

                              By:      /s/ J.K. Hage III
                                 ------------------------------
                              Name:   J.K. Hage III
                              Title:  Secretary

                              "OPTIONEE"
                                      /s/ Solon L. Kandel
                              ---------------------------------
                              Name:     Solon Kandel
                              Address:  592 Ashwood Road
                                        Springfield, New Jersey 07081
                                        Tel:  973-379-4902
                                        Fax:  973-379-1124

                              with a copy to:

                                        Steven B. Fuerst
                                        Lowenstein Sandler PC
                                        Post Office Plaza, Suite 504
                                        50 Division Street
                                        Somerville, New Jersey 08876-1113
                                        Tel:  908-526-3300
                                        Fax:  908-725-5124


                         Number of Discount Vested Option Shares:  11,235.9551
                         Number of Vested Option Shares:  16,853.9326
                         Number of Time Vesting Option Shares:  28,089.8876

                         Total Number of Option Shares:  56,179.7753