<PAGE>
                                                                    EXHIBIT 10.1
 
            FIRST AMENDMENT TO THE INTERCONNECTION AGREEMENT UNDER
          SECTIONS 251 AND 252 OF THE TELECOMMUNICATIONS ACT OF 1996


     This First Amendment ("the Amendment") to the Interconnection Agreement 
under Sections 251 and 252 of the Telecommunications Act of 1996 dated September
8, 1998 is entered between Ameritech Information Industry Services, a division 
of Ameritech Services, Inc., as an agent of Ameritech Illinois, with its 
principal offices at 350 North Orleans, Chicago, Illinois 60654 ("Ameritech"), 
and Focal Communication Corporation of Illinois, Inc. ("Focal") with its 
principal offices at 200 N. LaSalle Street, Suite 800, Chicago, Illinois 60601.

     WHEREAS, Ameritech and Focal are parties to that certain Interconnection 
Agreement under Sections 251 and 252 of the Telecommunications Act of 1996 dated
as of October 28, 1996 ("Agreement");

     WHEREAS, Focal is interested in purchasing Ameritech's Local Number 
Portability ("LNP") Database Access Service for the State of Illinois only.

     NOW, THEREFORE, in consideration of the mutual promises contained herein, 
Ameritech and Focal agree as follows.

1.0  DEFINITIONS
     
     Unless otherwise defined herein, capitalized terms shall have the meanings
assigned to such terms in the Agreement.

2.0  AMENDMENTS TO THE AGREEMENT

     On and after the Amendment Effective Date, the Agreement is hereby amended 
as follows:

     2.1  Insert the following new Section 5.5.2:

          "Each Party is responsible for requesting Interconnection to the other
          Party's CCIS network, where SS7 signaling on the trunk group(s) is
          desired. Each Party shall connect to a pair of access STPs that serve
          each LATA where traffic will be exchanged using a direct connection to
          the STPs serving the desired LATA, through the designated Ameritech
          state gateway STP or through a third party provider which is connected
          to the other Party's signaling network. The Party shall establish
          Interconnection at the STP. The rate for signaling links to establish
          such Interconnection is set

                                      -1-
<PAGE>
 
          forth in Ameritech FCC Tariff No. 2. If Focal does not possess STPs,
          Focal may purchase access to Ameritech's SS7 Network as set forth in
          Ameritech's FCC Tariff No. 2."

     2.2  Renumber Sections 5.5.2, 5.5.3 and 5.5.4 of the Agreement to Sections 
          5.5.3, 5.5.4, and 5.5.5 respectively.

     2.3  Add the following Section 13.6:

          "13.6 LNP Database Access Service.

          Ameritech shall provide Focal LNP LRN Information in accordance with 
          the provisions of Schedule 13.6."

3.0  MISCELLANEOUS

     3.1  The Agreement, as amended hereby, shall remain in full force and 
effect and each of the parties hereby ratifies and confirms its respective 
representations, warranties, covenants and agreements contained in and under the
Agreement. Any and all notices, requests, orders, certificates, documents and
execution and delivery of this Amendment may refer to the "Interconnection
Agreement under Sections 251 and 252 of the Telecommunications Act of 1996" or
may identify such Agreement in any other respect without making specific
reference to this Amendment, but nevertheless all such references shall be
deemed to include this Amendment unless the context shall otherwise require.

     3.2  This Amendment shall be deemed to be a contract made under and 
governed by the Rules and regulations of the Federal Communications Commission 
(FCC), as applicable, and the Telecommunications Act of 1996, and domestic laws 
of the State of Illinois, without reference to conflict of law provisions.

     3.3  This Amendment and all appendices attached hereto, constitutes the 
entire Amendment between the parties and supersedes all previous proposals, both
verbal and written.

     3.4  The Parties understand and agree that this Amendment may be superceded
by, or subject to the rules, regulations, or tariffs filed with or issued by the
FCC or may be subject to the approval of the FCC or the Commission as provided 
in Section 28.0 of the Agreement. In the event any inconsistency between the 
Agreement and such rules, regulations or tariffs, the rules, regulations and 
tariffs shall govern. If the Commission, the FCC or court rejects or supersedes 
any portion of this Amendment, the parties agree to meet and negotiate in good 
faith to arrive at a mutually acceptable modification of the rejected portion 
and related provisions; provided that such rejected portion shall not affect the
validity of the remainder of the Agreement.

                                      -2-
<PAGE>
 
Attached and incorporated herein are:

Schedule 13.6       -LNP (Local Number Portability) Database Access
                    Service

Appendix A          - Specifications and Standards
Appendix B          - Call Flow Diagram


IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be executed
by their duly authorized representatives.

Focal Communications Corporation
of Illinois
By: /s/ John R. Barnille
    ---------------------
Printed: John R. Barnille
         ----------------
Title:   EVP & C.O.O.
         ----------------
Date:    9/2/98
         ----------------

Ameritech Information Industry
Services
By: /s/ Karen S. Vessely
    ---------------------
Printed: Karen S. Vessely
         ----------------
Title:   President/AIIS
         ----------------
Date:    9/9/98
         ----------------

for Ameritech Services, Inc.
as an agent for Ameritech Illinois


                                      -3-
<PAGE>
 
                                 SCHEDULE 13.6
                                 -------------

1.   Definitions
     -----------

     A. LNP (Local Number Portability) is a technology that allows a end-user 
the ability to retain his/her telephone number when switching to a competing 
local telephone Service Provider.

     B. LNP Database Access Service allows carriers to query Ameritech's LNP 
database for LRN information in order to setup the network call routing to 
complete the call to a ported telephone number.

     C. SS7 (Common Channel Signaling System # 7) is signaling method in which a
single channel conveys, by means of labeled message, signaling information
relating to a multiplicity of circuits or calls and other information, such as
that used for network management.

     D. LRN (Location Routing Number) is a 10-digit number used to uniquely 
identify a switch that has ported numbers. LRN utilizes AIN and/or IN triggers, 
SS7 signaling, and unique 10-digit code for switch identification.

     E. Query(ies) is a signaling message requesting processing instructions or 
service data contained in a centralized database.

     F. Portable numbers are those telephone numbers within an exchange that 
have portable capabilities and are assigned to a designated portable exchange. 
These are the numbers that are assigned to the designated default switch and 
have not been ported to another Local Service Provider. These numbers are 
commonly identified as working on the "Donor" switch. Portable numbers are all 
numbers in an NXX where portability is allowed. Numbers will be declared 
portable on an NXX basis.

     G. Ported Numbers are those that have been assigned to other Local Service 
Providers providing recipient switch access for Portable exchanges where the 
user changes carriers while remaining in the same exchange. This is commonly 
termed as the numbers assigned to the "Recipient" switches. The "Recipient" 
switch is not the default switch in the SCP. Ported numbers are the subset of 
portable numbers that have actually been moved from the LERG based switch (donor
to another switch (recipient).

     H. N-1 Carrier, for Local Number Portability, is the originating or 
intermediate carrier that is delivering a call to the Ameritech switch for 
terminations and is responsible for determining the status and Location Routing 
Number of the dialed NXX.

                                      -4-
<PAGE>
 
     I. N-1 Responsibility is the N-1 Carrier who is required to perform to 
query calls to ported numbers before routing the call to the terminating 
carrier.
    
     J. AIN (Advanced Intelligent Network) is an evolving, service-independent 
network architecture that provides important new capabilities for rapid 
creation of customized telecommunications services. AIN offers one way to query 
a central database to obtain local number portability routing information.

     K. IN (Intelligent Network) is a hardware and software platform used to 
provide enhanced voice, video and data services. IN offers one way to query a 
central database to obtain local number portability routing information.

     L. Service Control Point (SCP) is a database in an SS7 network used to 
store LRN information.

     M. Switching Point (SP) means an SS7 Service capable of initiating and/or 
terminating SS7 messages. It may physically be an end office, an access tandem 
switch, an operator service system, a database manager, or other point.

     N. Service Switching Point (SSP) means the software capability within a 
Switching Point (SP) which provides the SP with SS7 message 
preparation/interpretation capability plus SS7 transmission/reception access 
ability.

     O. Signaling Transfer Point (STP) is the point where the customer 
interconnects with an SS7 network. In order to connect to Ameritech's SS7 
network, Customer-initiated LNP queries must connect with an Ameritech STP in 
order to connect to Ameritech's SCP.

2. Description of LNP Database Access Service
   ------------------------------------------

     A. Ameritech will provide to Focal LNP LRN information whenever Focal 
initiates a query for such information from an STP or SSP.

     B. All queries to Ameritech's LNP database shall use subsystem number (the 
designation of application) value of 0 with a translation type of 11 (AIN) 
and/or 201 (IN). Focal acknowledges that such subsystem number and translation
type values are necessary for Ameritech to properly process queries to its LNP
database.

     C. Focal warrants that it shall send queries conforming to the American 
National Standards Institute's (ANSI) approved standards for SS7 protocol and 
pursuant to the specification and standards identified in Appendix A. Focal 
acknowledges that transmission in said protocol is necessary for Ameritech to 
provision its LNP Database Access Service. Other applicable standards include

                                      -5-
<PAGE>
 
Bellcore specifications defining specific service applications, message types
and formats. Ameritech reserves the right to modify its network pursuant to
other specifications and standards that may become necessary to meet the
prevailing demands within the U.S. telecommunications industry. All such changes
shall be announced in advance.

     D. Focal acknowledges agrees to submit in advance of placing the order
LNP Database Access Service to submit a forecast of the volume of queries needed
to the Account Manager. The forecast will include the following items: the LATA
and/or MSA in which the queries will be initiated, Monthly Volume of LNP
queries, and the Busy Hour Volume of LNP queries. The forecast is required to
provide quality service levels. If the actual volumes of LNP queries exceed
120% of the forecasted volumes, the traffic may be subject to blockage to the
extent it is imperiling the reliability of LNP or causing congestion. In such
cases, Focal is required to submit a new forecast to the Account Manger.

     E. Focal acknowledges and agrees that SS7 network overload due to
extraordinary volumes of queries and/or other SS7 network messages can and will
have a detrimental effect on the performance of Ameritech's SS7 network. Focal
further agrees that Ameritech in its sole discretion, shall employ certain
automatic and/or manual overload controls within Ameritech's SS7 network to
guard against these detrimental effects. Ameritech shall report to Focal any
instances where overload controls are invoked to block traffic of Focal due to
Focal's SS7 network and Focal agrees in such cases to take immediate corrective
actions as are necessary to cure the conditions causing the overload situation.

     F. Focal acknowledges and agrees that any special testing considerations
must be submitted in writing to Focal's Account Manager. Special testing
arrangements must be negotiated for available time frames and dependent on
available Networks resources and testing options needed.

3. Price
   -----

     A. Ameritech will charge Focal the applicable Nonrecurring Charges for 
provisioning Focal's access to the Ameritech LNP Database based on Focal's 
current SS7 service. The following Nonrecurring Charges set forth in Ameritech 
FCC Tariff No. 2 shall apply:

          (1)  Administrative Charge,
                  per order
          (2)  Originating Point Code,
                  per OPC, per service,
                  added or changed,
                  per STP Pair      

                                      -6-


<PAGE>
 
          (3)  Global Title Address Translation,
                  per service,
                  added or changed,
                  per STP Pair

     B. Ameritech will charge Focal a per query rate of $.001169 for any query
initiated by Focal into Ameritech's LNP database for any information accessed.

     C. Ameritech will initially bill Focal in a manual format, until such time
as the mechanized billing system for LNP queries is implemented.

4.   Ownership of the LNP Database Information
     -----------------------------------------

     A. Ameritech retains full and complete ownership and control over the LNP
information in its database.

     B. Focal is prohibited from copying, storing, maintaining or creating any
database from any response it receives after initiating a LNP query to
Ameritech's LNP database.

5.   Termination of Service
     ----------------------

     A. Ameritech may terminate provision of this Service under this Amendment
at any time after February 28, 1999; provided, however Ameritech must provide
Focal sixty (60) days' prior written notice of such termination.

     B. If at any time during the term of this Agreement a state or federal
tariff for LNP Database Access Service becomes effective, the tariff and all
terms and conditions, including all rates, will supersede this Agreement. Under
these circumstances, no termination liability for either party will apply.

     C. Notwithstanding anything to the contrary in this Amendment, if legal or
regulatory decisions or rules compel Ameritech or Focal to terminate the
Amendment, Ameritech and Focal shall have no liability to the other in
connection with such termination. Termination under these circumstances shall be
deemed a termination without cause.

                                      -7-
<PAGE>
 
                                  Appendix A

Specifications and Standards

Description of Subject Area
and Issuing Organization                        Document Number
---------------------------                     ---------------

A. Bellcore, CCS Network Interface              TR-TSV-000905
      Specifications

B. Ameritech, Access to LNP Database            AM-TR-NIS-000145
                                                AM-TR-000146

C. Ameritech, Supplement Common Channel         AM-TR-OAT-000069
      Signaling (CCS)


                                      -8-       





<PAGE>
 
                                                                      Appendix B

                               Call Flow Diagram

The following diagram identifies the call flow for LNP Database Access Service.
The numbers on the diagram correspond to the description of the call flow as
noted in the diagram.

        [CALL FLOW DIAGRAM OF LNP DATABASE ACCESS SERVICE APPEARS HERE]

-------------------
----- Voice Traffic
===== Query Traffic
-------------------

NOTE: Redundant SS7 components and links not shown

Basic LNP Database Access Call Sequence

1) Focal's customer dials a number that has an NXX code that has been designated
   as number portable and the SSP office suspends the call and launches a query
   to the SCP. The query contains the Automatic Number Identification (ANI) type
   information and the called digits. The query transport path to the SCP will
   depend on the type of SSP interconnection.

2) The SCP's service logic performs the necessary analysis of the ten digit
   called number, determines the appropriate routing instructions, and returns a
   response to the originating SSP. If the number has been ported, the Location
   Routing Number ("LRN") of the recipient switch that now serves the number is
   returned as well as the called number.

3) Focal uses the routing information to resume call processing and then
   completes the call over the appropriate facility, such as an Ameritech Tandem
   Switch.
                                      -9-