<PAGE>   1
                                                                  EXHIBIT 10.50


[*] - Confidential treatment requested pursuant to Rule 24b-2.

AT&T COMMUNICATIONS                                   CONTRACT TARIFF NO. 13567
Adm. Rates and Tariffs                                      Original Title Page
Bridgewater, NJ 08807
Issued:  March 22, 2000                              Effective:  March 23, 2000

                    ** All material on this page is new. **


                           CONTRACT TARIFF NO. 13567

                                   TITLE PAGE

This Contract Tariff applies to AT&T InterSpan Frame Relay Service; AT&T
Private Line Services, AT&T International Satellite Services and AT&T Local
Channel Services for interstate or foreign communications in accordance with
the Communications Act of 1934, as amended.

Telecommunication services provided under this Contract Tariff are furnished by
means of wire, radio, satellite, fiber optics or any suitable technology or
combination of technologies.


<PAGE>   2


AT&T COMMUNICATIONS                                   CONTRACT TARIFF NO. 13567
Adm. Rates and Tariffs                                          Original Page 1
Bridgewater, NJ 08807
Issued:  March 22, 2000                              Effective:  March 23, 2000

                    ** All material on this page is new. **

                           CONTRACT TARIFF NO. 13567

                                  CHECK SHEET

The Title Page and Pages 1 through 11 inclusive of this tariff are effective as
of the date shown.

                               TABLE OF CONTENTS
<TABLE>
<CAPTION>
                                                                                                                  Page
                                                                                                                  ----
<S>                                                                                                               <C>
Check Sheet.......................................................................................................   1
List of Concurring, Connecting and Other Participating Carriers...................................................   1
Explanation of Symbols - Coding of Tariff Revisions...............................................................   1
Trademarks and Service Marks......................................................................................   2
Explanation of Abbreviations......................................................................................   2
General Provisions................................................................................................   2
Contract Summary..................................................................................................   4
</TABLE>

LIST OF CONCURRING, CONNECTING AND OTHER PARTICIPATING CARRIERS

CONCURRING CARRIERS - NONE

CONNECTING CARRIERS - NONE

OTHER PARTICIPATING CARRIERS - NONE

EXPLANATION OF SYMBOLS - Coding of Tariff Revisions

Revisions to this tariff are coded through the use of symbols. These symbols
appear in the right margin of the page. The symbols and their meanings are:

         R        - to signify reduction.
         I        - to signify increase.
         C        - to signify changed regulation.
         T        - to signify a change in text but no change in rate or
                    regulation.
         S        - to signify reissued matter.
         M        - to signify matter relocated without change.
         N        - to signify new rate or regulation.
         D        - to signify discontinued rate or regulation.
         Z        - to signify a correction.

Other marginal codes are used to direct the tariff reader to a footnote for
specific information. Codes used for this purpose are lower case letters of the
alphabet, e.g., x, y and z. These codes may appear beside the page revision
number in the page header or in the right margin opposite specific text.


<PAGE>   3


AT&T COMMUNICATIONS                                   CONTRACT TARIFF NO. 13567
Adm. Rates and Tariffs                                          Original Page 2
Bridgewater, NJ 08807
Issued:  March 22, 2000                              Effective:  March 23, 2000

                    ** All material on this page is new. **

TRADEMARKS AND SERVICE MARKS - The following marks, to the extent, if any, used
throughout this tariff, are trademarks and service marks of AT&T Corp.

<TABLE>
<CAPTION>
                    Trademarks            Service Marks
                    ----------            -------------

                    <S>                   <C>
                       None                  ACCUNET

                                            DATAPHONE

                                            InterSpan
</TABLE>

EXPLANATION OF ABBREVIATIONS

Adm.              - Administrator
ASDS              - ACCUNET Spectrum of Digital Access
GDA               - Generic Digital Access
kbps              - kilobits per second
Mbps              - Megabits per second

                               GENERAL PROVISIONS

I.       CUSTOMER'S INITIAL SERVICE DATE - The date on which the term of this
Contract Tariff begins is referred to as the Customer's Initial Service Date
(CISD). The rates and discounts specified in this Contract Tariff will apply
commencing at the CISD. The CISD is the date that the Customer begins service
under this Contract Tariff.


<PAGE>   4


AT&T COMMUNICATIONS                                   CONTRACT TARIFF NO. 13567
Adm. Rates and Tariffs                                          Original Page 3
Bridgewater, NJ 08807
Issued:  March 22, 2000                              Effective:  March 23, 2000

                    ** All material on this page is new. **

                           CONTRACT TARIFF NO. 13567

1.       SERVICES PROVIDED:

         A.       AT&T InterSpan Frame Relay Service (FRS) (AT&T Tariff F.C.C.
                  No. 4)
         B.       AT&T Private Line Services (AT&T Tariff F.C.C. No. 9)
         C.       AT&T Local Channel Services (AT&T Tariff F.C.C. No. 11)
         D.       AT&T International Satellite Services (AT&T Tariff F.C.C.
                  No. 7)

2.       CONTRACT TERM; RENEWAL OPTIONS - The term of this Contract Tariff (CT)
         is 4 years. No renewal option is available for this CT.


<PAGE>   5


AT&T COMMUNICATIONS                                   CONTRACT TARIFF NO. 13567
Adm. Rates and Tariffs                                          Original Page 4
Bridgewater, NJ 08807
Issued:  March 22, 2000                              Effective:  March 23, 2000

                    ** All material on this page is new. **

3.       MINIMUM COMMITMENTS/CHARGES

         A.       AT&T PRIVATE LINE, AT&T INTERSPAN FRAME RELAY AND AT&T LOCAL
CHANNEL SERVICES - The combined Data Minimum Annual Revenue Commitment (DMARC)
for the AT&T Private Line, FRS, International Satellite and AT&T Local Channel
Services provided under this CT is as follows:

<TABLE>
<CAPTION>
CT TERM YEAR           YEAR 1          YEAR 2           YEAR 3           YEAR 4
<S>                  <C>             <C>             <C>              <C>
DMARC                $8,000,000      $9,000,000      $11,000,000      $13,000,000
</TABLE>


The DMARC will be satisfied by the undiscounted recurring charges for Digital
Services Volume Pricing Plan (DSVPP) - eligible service components, as
specified in AT&T Tariff F.C.C. Nos. 4, 9 and 11, as amended from time to time,
(excluding those AT&T Terrestrial 1.544 Mbps Local Channels provided under the
Access Value Arrangement (AVA) with the Universal Terrestrial 1.544 Mbps Local
Channels Access Arrangement AVA/UTA of Section 5.D., following) and by the
undiscounted recurring charges for International Private Line service
components consisting of: International ACCUNET Digital Services-Half Channels,
International Full Channel Service-Overseas-Overseas Cable Digital Channels,
and Voice Grade Services-Overseas Half Channels as specified in AT&T Tariff
F.C.C. No. 9, as amended from time to time, and by the undiscounted recurring
charges for the AT&T International Satellite Shared Earth Station Service-Half
Channels as specified in AT&T F.C.C. Tariff No. 7, as amended from time to
time, and by undiscounted Monthly Charges for FRS Domestic Access Ports as
specified in Section 7., following, and alternative data communications
capabilities which have been ordered under a separate contract between the
Customer and AT&T and which have been mutually designated in writing by the
parties prior to or during the term of this Contract Tariff, for the Services
Provided under this CT. If, on any anniversary of the CISD, the Customer has
failed to satisfy the DMARC for the preceding year, the Customer will be billed
a shortfall charge in an amount equal to the difference between the DMARC and
the sum of: (1) the actual undiscounted recurring charges for the
DSVPP-eligible service components in service for that year under this CT, (2)
the actual undiscounted recurring charges for AT&T International Private Line
and International Satellite service components, (3) the actual undiscounted
Monthly Charges for the FRS Domestic Access Ports in service for that year
under this CT and (4) the actual undiscounted recurring charges for alternative
data communications capabilities.


<PAGE>   6


AT&T COMMUNICATIONS                                   CONTRACT TARIFF NO. 13567
Adm. Rates and Tariffs                                          Original Page 5
Bridgewater, NJ 08807
Issued:  March 22, 2000                              Effective:  March 23, 2000

                    ** All material on this page is new. **

4.       CONTRACT PRICE

         A.       The Contract Price for the AT&T Services provided under this
CT is the same as the undiscounted Recurring and Nonrecurring Rates and Charges
specified in AT&T Tariffs listed in Section 1., preceding, as amended from time
to time, except for those Rates specified in Section 7., following.

5.       DISCOUNTS - The following discounts are the only discounts that apply
to the Services Provided under this CT. The Customer will receive the following
discounts, each month, in lieu of any other discounts. These discounts will be
applied to the undiscounted recurring charges for DSVPP-eligible service
components for FRS and AT&T Private Line Services in service under this CT and
to the undiscounted monthly charges for FRS Domestic Access Ports in service
under this CT in the same manner as the DSVPP discounts as specified in AT&T
Tariff F.C.C. Nos. 4, 9 and 11, as amended from time to time.

         A.       AT&T INTERSPAN FRAME RELAY SERVICES AND FRS DOMESTIC ACCESS
PORTS - The Customer will receive a discount of *, each month.

         B.       AT&T PRIVATE LINE SERVICES - The Customer will receive the
following discounts, each month.

<TABLE>
<CAPTION>
                                                                   Discount %
                                                                   ----------
         <S>                                                       <C>
         ASDS at speeds of 64 kbps and below*                           *
         ASDS at speeds of 128 kbps and above*                          *
         ACCUNET T1.5 Services*                                         *
         ACCUNET T32 Services                                           *
         ACCUNET T45 Services*                                          *
         ACCUNET Fractional T45 Services                                *
         DATAPHONE Digital Services*                                    *
         International ACCUNET 2.048 Mbps Service-Mexico                *
         ACCUNET SONET T155 Services*                                   *
         SONET OC12 Services                                            *
</TABLE>

* - Includes AT&T Tariff F.C.C. No. 9 Canada and Mexico Services.

         C.       AT&T LOCAL CHANNEL SERVICES - The Customer will receive the
following discounts, each month.

<TABLE>
<CAPTION>
                                                                             Discount %
                                                                             ----------
         <S>                                                                 <C>
         9.6/56/64 ACCUNET Generic Digital Access Services                       *
         AT&T Terrestrial 1.544 Mbps Local Channel Services                      *
            (excluding the components in Section 5.D., following)
         Voice Grade Local Channel Services                                      *
         9.6/56/64 kbps Digital Data Local Channel Services                      *
         AT&T  Terrestrial 45 Mbps Local Channels and associated                 *
         Access Coordination Functions
</TABLE>


<PAGE>   7


AT&T COMMUNICATIONS                                   CONTRACT TARIFF NO. 13567
Adm. Rates and Tariffs                                          Original Page 6
Bridgewater, NJ 08807
Issued:  March 22, 2000                              Effective:  March 23, 2000

                    ** All material on this page is new. **

5.       DISCOUNTS (CONTINUED)

         D.       AT&T TERRESTRIAL 1.544 MBPS LOCAL CHANNELS - *.

         E.       AT&T INTERNATIONAL SATELLITE AND INTERNATIONAL AT&T PRIVATE
LINE SERVICES - The Customer will receive the following discounts each month on
the undiscounted recurring charges for service components for AT&T
International Satellite Service and International AT&T Private Line Services
provided under this CT. The below discounts will be applied in the same manner
as DSVPP discounts specified in AT&T Tariff F.C.C. No. 9, even though such
services are not DSVPP-eligible.

<TABLE>
<CAPTION>
         Service Components                                                                   Discount
         ------------------                                                                   --------
         <S>                                                                                  <C>
         Voice Grade Private Line Service-Overseas Half Channel                                    *
         International ACCUNET Digital Services-Half Channel                                       *
         International Satellite Shared Earth Station Service-Half Channel                         *
         International Full Channel Service-Overseas-Overseas Cable Digital Channel                *
</TABLE>

6.       CLASSIFICATIONS, PRACTICES AND REGULATIONS

         A.       Except as otherwise provided in this CT, the rates and
regulations that apply to the Services Provided specified in Section 1.,
preceding, are as set forth in the AT&T Tariffs that are referenced in Section
1., preceding, as such tariffs are amended from time to time.

         B.       MONITORING CONDITIONS - None.


<PAGE>   8


AT&T COMMUNICATIONS                                   CONTRACT TARIFF NO. 13567
Adm. Rates and Tariffs                                          Original Page 7
Bridgewater, NJ 08807
Issued:  March 22, 2000                              Effective:  March 23, 2000

                    ** All material on this page is new. **

6.       CLASSIFICATIONS, PRACTICES AND REGULATIONS (CONTINUED)

         C.       PROMOTIONS, CREDITS AND WAIVERS - The following credits and
waivers will be applied to the Customer's bill subject to the following
limitations: (1) all credits and waivers apply only to the Private Line, Local
Channel and InterSpan Frame Relay Services provided under this CT and as
specified below; (2) any waiver not applied by the end of the CT will be null
and void; (3) installation and monthly charge waivers apply only to new service
components (unless otherwise specified below) and do not apply to service
components disconnected and reconnected after the CISD; (4) the service
components must remain in service for a minimum retention period of 12 months
(unless otherwise specified below); and (5) the credits/waivers under this
section do not apply to Network Protection Capability (NPC). If any of the
installed services components are disconnected prior to the end of the minimum
retention period, AT&T will bill the Customer for the amount of the charges
that had been waived, or the amount of the credit that had been applied equal
to installation charges, under this section for each service component
disconnected. Any such bill must be paid by the Customer within 30 days.

         1.       NONRECURRING CHARGES - The following charges, as specified in
AT&T Tariffs listed in Section 1., preceding, as amended from time to time, are
waived.

                  (a)      The Installation Charges for DSVPP-eligible service
components and associated Functions Connections, those Local Channels specified
in Section 5.D., preceding, and FRS Domestic Access Ports as specified in
Section 7., following. If ACCUNET SONET T155 Service and SONET OC12 Service IOC
and Multiplexing Office Function service components are not in service for 12
months, the Customer will also be billed the termination charge as specified in
AT&T Tariff F.C.C. No. 9, as amended from time to time.

                  (b)      The Installation Charges for Office Connections
associated with the International Channel types specified in Section 5.E.,
preceding.

         2.       RECURRING CHARGES - The following charges, as specified in
AT&T Tariffs listed in Section 1., preceding, as amended from time to time, are
waived.

                  (a)      The recurring charges for the following new and
existing service components are waived: (1) ACCUNET T1.5 Access Connections and
Access Coordination Functions associated with AT&T Terrestrial 1.544 Mbps Local
Channels; (2) ACCUNET T1.5 M-24 Multiplexing Office Functions; and (3) ASDS
56/64 kbps Access Connections, Access Coordination Functions associated with
56/64 kbps ACCUNET GDA Services. There is no minimum retention period
associated with this waiver.


<PAGE>   9


AT&T COMMUNICATIONS                                   CONTRACT TARIFF NO. 13567
Adm. Rates and Tariffs                                          Original Page 8
Bridgewater, NJ 08807
Issued:  March 22, 2000                              Effective:  March 23, 2000

                    ** All material on this page is new. **

6.C.     PROMOTIONS, CREDITS AND WAIVERS (CONTINUED)

         3.       AT&T will apply a credit equal to the installation charges, as
specified in AT&T Tariff F.C.C. No. 11, as amended from time to time, for the
installation of new AT&T Terrestrial 45 Mbps Local Channels subscribed to under
an AVP.

         4.       AT&T will apply a credit against charges billed under this
Contract Tariff equal to the installation charges incurred by the Customer for
AT&T Regional Frame Relay Service DSVPP-eligible service components during the
Contract Tariff Term.

         5.       AT&T will apply a credit of * in the 3rd month following the
CISD to the Customer's Frame Relay Service bill. If the Customer discontinues
this CT for any reason prior to the expiration of the CT Term and incurs a
Termination Charge as specified in Section 6.D., following, AT&T will bill the
Customer, at the time of discontinuance, the entire amount of the credit
received. Any such bill must be paid by the Customer within 30 days.

         6.       AT&T will apply a credit of * in the 4th month following the
CISD to the Customer's Frame Relay Service bill. If the Customer discontinues
this CT for any reason prior to the expiration of the CT Term and incurs a
Termination Charge as specified in Section 6.D., following, AT&T will bill the
Customer, at the time of discontinuance, the entire amount of the credit
received. Any such bill must be paid by the Customer within 30 days.

         D.       DISCONTINUANCE - In lieu of any Discontinuance With or Without
Liability provisions that are specified in the AT&T Tariffs referenced in
Section 1., preceding, the following provisions shall apply.

The Customer may discontinue this CT prior to the end of the CT Term, provided
the Customer replaces this CT with other AT&T Tariffed Interstate Services or
another AT&T CT for AT&T Tariffed Interstate Services having: (i) an equal or
greater new DMARC, and (ii) a new term equal to or greater than the existing
term. However, the Customer will be billed a Shortfall Charge equal to the
differences between: (1) the prorated DMARC for the year in which the Customer
discontinues, and (2) the total of the actual undiscounted recurring charges
used to satisfy the corresponding DMARC for that year under this CT, provided
the amount in (2) is less than the amount in (1).


<PAGE>   10


AT&T COMMUNICATIONS                                   CONTRACT TARIFF NO. 13567
Adm. Rates and Tariffs                                          Original Page 9
Bridgewater, NJ 08807
Issued:  March 22, 2000                              Effective:  March 23, 2000

                    ** All material on this page is new. **

6.D.     DISCONTINUANCE (CONTINUED)

If the Customer discontinues this CT for any reason other than specified above,
prior to the expiration of the CT Term, a Termination Charge will apply. The
Termination Charge will be an amount equal to 35% of the sum of the unsatisfied
DMARCs for the year in which the Customer discontinues this CT and 35% of the
sum of the DMARCs for each year remaining in the CT Term. This Termination
Charge is in addition to any Termination Charge for service components
disconnected prior to the end of the minimum retention period, if applicable.
The Customer will also be billed an amount as specified in Section 6.c.5. and
6.c.6., preceding.

         E.       OTHER REQUIREMENTS - Not Applicable.

         F.       AVAILABILITY - This CT is available only to Customers who:
(1) will order this CT only once, either by the Customer or any Affiliate of
the Customer, which is any entity that owns a controlling interest in either
the Customer or an Affiliate of the Customer, or any entity in which a
controlling interest is owned by either the Customer or an Affiliate of the
Customer; and (2) order service within 30 days after the effective date of this
CT for initial installation of the Services Provided under this CT within 30
days after the date ordered.


<PAGE>   11


AT&T COMMUNICATIONS                                   CONTRACT TARIFF NO. 13567
Adm. Rates and Tariffs                                         Original Page 10
Bridgewater, NJ 08807
Issued:  March 22, 2000                              Effective:  March 23, 2000

                    ** All material on this page is new. **

7.       RATES

         A.       AT&T INTERSPAN FRAME RELAY SERVICE - The below rates are
stabilized for the CT Term.

         1.       PORTS

                               PORT CHARGES TABLE

<TABLE>
<CAPTION>
                              Port Speed kbps       Domestic Port Monthly Charge         Port Installation Charge
                              ---------------       ----------------------------         ------------------------
                              <S>                   <C>                                  <C>
                                     56                           *                                   *
                                     64                           *                                   *
                                    128                           *                                   *
                                    192                           *                                   *
                                    256                           *                                   *
                                    320                           *                                   *
                                    384                           *                                   *
                                    448                           *                                   *
                                    512                           *                                   *
                                    576                           *                                   *
                                    640                           *                                   *
                                    704                           *                                   *
                                    768                           *                                   *
                                   1544                           *                                   *
</TABLE>

         2.       FRS DOMESTIC ACCESS PORT CHARGES - When the service components
specified below are ordered together as a unit at the same location, the
Customer will be billed the following in lieu of the individual service
component charges:

                  (a)      Domestic Access Port (a Domestic Access Port consists
of one Domestic Port, one Digital Local Channel, one Access Connection and one
Access Coordination Function). The Digital Local Channel can only be used to
send FRS traffic to the Domestic Port and for no other purpose. Domestic Access
Ports are only available in the Mainland and Hawaii.

<TABLE>
<CAPTION>
    Domestic Access Port           Monthly Charge           Installation Charge
    --------------------           --------------           -------------------
    <S>                            <C>                      <C>
           56 kbps                       *                           *
</TABLE>


<PAGE>   12


AT&T COMMUNICATIONS                                   CONTRACT TARIFF NO. 13567
Adm. Rates and Tariffs                                         Original Page 11
Bridgewater, NJ 08807
Issued:  March 22, 2000                              Effective:  March 23, 2000

                    ** All material on this page is new. **

7.A.     AT&T INTERSPAN FRAME RELAY SERVICE

         3.       PERMANENT VIRTUAL CIRCUITS

                           DOMESTIC PVC CHARGES TABLE

<TABLE>
<CAPTION>
                                        Symmetrical
                 PVC CIR                 PVC Monthly
                  kbps                     Charge
                 ------                 ------------
                 <S>                    <C>
                    4                        *
                    8                        *
                   16                        *
                   32                        *
                   48                        *
                   56                        *
                   64                        *
                  128                        *
                  192                        *
                  256                        *
                  320                        *
                  384                        *
                  448                        *
                  512                        *
                  576                        *
                  640                        *
                  704                        *
                  768                        *
                  832                        *
                  896                        *
                  960                        *
                 1024                        *
                 1536                        *
</TABLE>

         B.       AT&T LOCAL CHANNEL SERVICES - The below rates are stabilized
for the CT Term.

         1.       ACCUNET T1.5 LOCAL CHANNEL

<TABLE>
<CAPTION>
                                                   MONTHLY
                                       ---------------------------------                 INSTALLATION
          MILEAGE                      FIXED                    PER MILE                    CHARGE
          -------                      -----                    --------                    ------
          <S>                          <C>                      <C>                      <C>
             0                           *                         N/A                         *
           over 0                        *                         N/A                         *
</TABLE>