<PAGE>

                                                                    Exhibit 10.8


                                  TRITON PCS

                                   Exhibit A
                                SERVICE CHARGES


Service Rates - AT&T shall pay Triton
-------------------------------------

   8/16/99-5/31/00  $.25 per minute or partial minute
   6/1/00-1/31/01   $.20 per minute or partial minute
   2/1/01-1/31/02   $.15 per minute or partial minute
   2/1/02-1/31/03   $.13 per minute or partial minute
   2/1/03-8/31/03   $.12 per minute or partial minute
   9/1/03-8/31/05   $.10 per minute or partial minute
   9/1/05-          The lower of (a) $.10 per minute and (b) the Applicable Home
                    Rate of AT&T, or such other rate as shall be agreed by the
                    Parties.

Service Rates - Triton shall pay AT&T
-------------------------------------

   8/16/99-8/31/03  $.10 per minute or partial minute
   9/1/03-          Reciprocal rates as shall be charged between AT&T and
                    Triton.

The Parties agree to exclude Tritel, Cincinnati Bell Wireless and Telecorp
markets from the rates listed above.  The rates between these entities and
Triton shall be:

Service Rates - Tritel (Reciprocal)
-----------------------------------

   8/16/99-8/31/05  $.10 per minute or partial minute
   9/1/05-          The lower of (a) $.10 per minute and (b) the Applicable Home
                    Rate of AT&T, or such other rate as shall be agreed by the
                    Parties.

Service Rates - Cincinnati Bell Wireless (Reciprocal)
-----------------------------------------------------

   8/16/99-8/31/05  $.15 per minute or partial minute
   9/1/05-          The lower of (a) $.10 per minute and (b) the Applicable Home
                    Rate of AT&T, or such other rate as shall be agreed by the
                    Parties.


Service Rates - Telecorp (Reciprocal)
-------------------------------------

   8/16/99-         $.25 per minute or partial minute

A reduction in the Service Rate between Telecorp and Triton shall be negotiated
in good faith as soon as soon as commercially reasonable, and shall be
reasonably competitive with the rates charged for comparable service.


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Notwithstanding the foregoing, the Service rate charged by the Parties for
Service in any geographic area consisting of at least three contiguous BTAs
shall be no more than reasonably competitive, taking into account price,
coverage and quality, with the rates charged for comparable telecommunications
services by the alternative carriers in such geographic area, disregarding any
carriers that do not service a material number of customers in such geographic
area, and, if it is not so competitive, such Service Rates shall be reduced with
respect to the applicable geographic area to a reasonably competitive rate.

Toll Charges
------------

InterLATA
---------
               $.08 per minute or such other rate as the Parties negotiate from
               time to time in light of cost adjustments.

IntraLATA
---------
               $.02 per minute or such other rate as the Parties negotiate from
               time to time in light of cost adjustments.

Taxes
-----

Amount charged Serving Carrier by applicable taxing authority.


Miscellaneous
-------------

Service rates are charged in full minute increments with each partial minute
rounded to the next full minute.  Neither Party shall be charged for incomplete
calls, busy calls, feature activations, 911, 611, #611 or interconnect fees.

The Home Carrier will have up to 90 days after the end of a settlement period to
invoice the Serving Carrier for roaming rates charged in excess of the agreed
upon roaming rates during that settlement period.

This Service Charges Exhibit will supercede and replace all previous roaming
rate agreements between AT&T and Triton PCS.


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Definitions
-----------

"Applicable Home Rate" means, with respect to any Person, its Home Rate,
 --------------------
calculated based on the revenues from access and airtime and the minutes of use,
in each case for such Person's customers in the Expanded Home Calling Area
during the most recent calendar year for which such amounts are available as of
the date of calculation.

"Expanded Home Calling Area" means the geographical area of, and markets
 --------------------------
included within, the Charlotte, North Carolina, Atlanta, Georgia, Baltimore,
Maryland/Washington, D.C., and Richmond, Virginia, MTAs, and the State of
Florida.


AT&T WIRELESS SERVICES, INC.               TRITON PCS, INC.




By:       /s/ Don Adams                    By:         /s/ Drew Davies
      ----------------------------               -------------------------------
       Don Adams, Vice President                   Drew Davies, Vice President
       Carrier Relations                           Carrier Relations


Date:      8/11/99                         Date:        8/11/99
      ----------------------------               -------------------------------




Confidential                        Page 3                              08/11/99