<PAGE>

                                                                     Exhibit 4.1

                                ACQUISITION LOAN
                                 PROMISSORY NOTE

$2,600,000.00                                                  December 22, 2006
                                                           Indianapolis, Indiana

     1. AGREEMENT TO PAY. FOR VALUE RECEIVED, CTI GROUP (HOLDINGS), INC., a
Delaware corporation ("BORROWER"), hereby promises to pay to the order of
NATIONAL CITY BANK, a national banking association, its successors and assigns
("LENDER"), the principal sum of Two Million Six Hundred Thousand and No/100
Dollars ($2,600,000.00) ("LOAN"), or so much thereof as may be advanced pursuant
to that certain Loan Agreement of even date herewith between Borrower and Lender
("LOAN AGREEMENT") at the place and in the manner hereinafter provided, together
with interest thereon at the rate or rates described below, and any and all
other amounts which may be due and payable hereunder from time to time without
relief from valuation or appraisement laws.

     2. INTEREST RATE. Interest shall accrue on the outstanding principal
balance of this Note from the date hereof through the Acquisition Loan Maturity
Date (as defined in the Loan Agreement) at the interest rate as set forth in the
Loan Agreement.

     3. PAYMENT TERMS.

          3.1 PRINCIPAL AND INTEREST. Payments of principal and interest due
     under this Note, if not sooner declared to be due in accordance with the
     provisions hereof, shall be made as follows:

               (a) Payment of interest shall be made as set forth in the Loan
          Agreement.

               (b) The unpaid principal balance of this Note, if not sooner paid
          or declared to be due in accordance with the terms hereof or the terms
          of the Loan Agreement, together with all accrued and unpaid interest
          thereon and any other amounts due and payable hereunder or under any
          other Loan Document (as hereinafter defined), shall be due and payable
          in full at the Acquisition Loan Maturity Date.

          3.2 APPLICATION OF PAYMENTS. Prior to the occurrence of an Event of
     Default, all payments and prepayments on account of the indebtedness
     evidenced by this Note shall be applied as follows: (a) first, to fees,
     expenses, costs and other similar amounts then due and payable to Lender,
     including, without limitation any prepayment premium, exit fee or late
     charges due hereunder, (b) second, to accrued and unpaid interest on the
     principal balance of this Note, (c) third, to the payment of principal due
     in the month in which the payment or prepayment is made, (d) fourth, to any
     escrows, impounds or other amounts which may then be due and payable under
     the Loan Documents, (e) fifth, to any other amounts then due Lender
     hereunder or under any of the Loan Documents, and (f) last, to the unpaid
     principal balance of this Note. Any prepayment on account of the

<PAGE>

     indebtedness evidenced by this Note shall not extend or postpone the due
     date or reduce the amount of any subsequent payment of principal or
     interest due hereunder. After an Event of Default has occurred and is
     continuing, payments may be applied by Lender to amounts owed hereunder and
     under the Loan Documents in such order as Lender shall determine, in its
     sole discretion.

          3.3 METHOD OF PAYMENTS. All payments of principal and interest
     hereunder shall be paid by automatic debit, wire transfer, check or in coin
     or currency which, at the time or times of payment, is the legal tender for
     public and private debts in the United States of America and shall be made
     at such place as Lender or the legal holder or holders of this Note may
     from time to time appoint in the payment invoice or otherwise in writing,
     and in the absence of such appointment, then at the offices of Lender at
     One National City Center, Suite 200E, Indianapolis, Indiana 46255. Payment
     made by check shall be deemed paid on the date Lender receives such check;
     provided, however, that if such check is subsequently returned to Lender
     unpaid due to insufficient funds or otherwise, the payment shall not be
     deemed to have been made and shall continue to bear interest until
     collected. Notwithstanding the foregoing, the final payment due under this
     Note must be made by wire transfer or other final funds. If requested by
     Borrower, interest, principal payments and any fees and expenses owed
     Lender from time to time will be deducted by Lender automatically on the
     due date from Borrower's account with Lender, as designated in writing by
     Borrower. Borrower will maintain sufficient funds in the account on the
     dates Lender enters debits authorized by this Note. If there are
     insufficient funds in the account on the date Lender enters any debit
     authorized by this Note, the debit will be reversed. Borrower may terminate
     this direct debit arrangement at any time by sending written notice to
     Lender at the address specified in the Loan Agreement.

          3.4 LATE CHARGE. If any payment of interest or principal due hereunder
     is not made within ten (10) days after such payment is due in accordance
     with the terms hereof, then, in addition to the payment of the amount so
     due, Borrower shall pay to Lender a "late charge" of the greater of: (i)
     five cents for each whole dollar so overdue or (ii) Twenty-Five Dollars
     ($25.00) to defray part of the cost of collection and handling such late
     payment. Borrower agrees that the damages to be sustained by the holder
     hereof for the detriment caused by any late payment are extremely difficult
     and impractical to ascertain, and that the amount of five cents for each
     one dollar due is a reasonable estimate of such damages, does not
     constitute interest, and is not a penalty.

     4. SECURITY. This Note is secured by (i) the Loan Agreement, (ii) the
Guaranties (the "U.S. GUARANTIES") of even date herewith from CTI Data Solutions
(USA) Inc., CTI Billing Solutions, Inc., CTI Delaware Holdings, Inc., Centillion
Data Systems, L.L.C., CTI Data Solutions Ltd, CTI Billings Solutions Ltd and
Ryder Systems Ltd to Lender, (iii) the Security Agreements ("U.S. SECURITY
AGREEMENTS") of even date herewith from Borrower, CTI Data Solutions (USA) Inc.,
CTI Billing Solutions, Inc., CTI Delaware Holdings, Inc. and Centillion Data
Systems, L.L.C. to Lender, (iv) the Charge Over Shares In Ryder Systems Ltd from
Ryder Systems Ltd to Lender of even date herewith (the "CHARGE OVER SHARES"),
(v) the Debentures between the Borrower, CTI Data Solutions Ltd and Ryder
Systems Ltd, and Lender of even date herewith (the "UK DEBENTURES"), and (vi)
the Guarantee and Indemnities between CTI Data


                                       2

<PAGE>

Solutions Ltd and Ryder Systems Ltd, and Lender of even date herewith (the "UK
GUARANTEES") (the Loan Agreement, the U.S. Guaranties, the U.S. Security
Agreements, the Charge Over Shares, the UK Debentures, the UK Guarantees and any
other document now or hereafter given to evidence or secure payment of this Note
or delivered to induce Lender to disburse the proceeds of the Loan, as such
documents may hereafter be amended, restated or replaced from time to time, are
hereinafter collectively referred to as the "LOAN DOCUMENTS"). Reference is
hereby made to the Loan Documents (which are incorporated herein by reference as
fully and with the same effect as if set forth herein at length) for a statement
of the covenants and agreements contained therein, a statement of the rights,
remedies, and security afforded thereby, and all matters therein contained.

     5. EVENTS OF DEFAULT. The occurrence of any "Event of Default" (after the
expiration of any applicable notice and/or cure periods) under any of the Loan
Documents shall constitute an "EVENT OF DEFAULT" under this Note.

     6. REMEDIES. At the election of the holder hereof, and after written notice
to Borrower, the principal balance remaining unpaid under this Note, and all
unpaid interest accrued thereon and any other amounts due hereunder, shall be
and become immediately due and payable in full upon the occurrence and during
the continuance of any Event of Default. Failure to exercise this option shall
not constitute a waiver of the right to exercise same in the event of any
subsequent Event of Default. No holder hereof shall, by any act of omission or
commission, be deemed to waive any of its rights, remedies or powers hereunder
or otherwise unless such waiver is in writing and signed by the holder hereof,
and then only to the extent specifically set forth therein. The rights, remedies
and powers of the holder hereof, as provided in this Note, the Loan Agreement
and in all of the other Loan Documents are cumulative and concurrent, and may be
pursued singly, successively or together against Borrower, any guarantor
thereof, the security given at any time to secure the repayment hereof, all at
the sole discretion of the holder hereof. If any suit or action is instituted or
attorneys are employed to collect this Note or any part hereof, Borrower
promises and agrees to pay all costs of collection, including reasonable
attorneys' fees and court costs.

     7. COVENANTS AND WAIVERS. Borrower and all others who now or may at any
time become liable for all or any part of the obligations evidenced hereby,
expressly agree hereby to be jointly and severally bound, and jointly and
severally: (i) waive and renounce any and all homestead, redemption and
exemption rights and the benefit of all valuation and appraisement privileges
against the indebtedness evidenced by this Note or by any extension or renewal
hereof; (ii) waive presentment and demand for payment, notices of nonpayment and
of dishonor, protest of dishonor, and notice of protest; (iii) waive any and all
notices in connection with the delivery and acceptance hereof and all other
notices in connection with the performance, default, or enforcement of the
payment hereof or hereunder; (iv) waive any and all lack of diligence and delays
in the enforcement of the payment hereof; (v) agree that the liability of
Borrower, guarantor, endorser or obligor shall be unconditional and without
regard to the liability of any other person or entity for the payment hereof,
and shall not in any manner be affected by any indulgence or forbearance granted
or consented to by Lender to any of them with respect hereto; (vi) consent to
any and all extensions of time, renewals, waivers, or modifications that may be
granted by Lender with respect to the payment or other provisions hereof, and to
the release of any security at any time given for the payment hereof, or any
part thereof, with or without


                                       3

<PAGE>

substitution, and to the release of any person or entity liable for the payment
hereof; and (vii) consent to the addition of any and all other makers,
endorsers, guarantors, and other obligors for the payment hereof, and to the
acceptance of any and all other security for the payment hereof, and agree that
the addition of any such makers, endorsers, guarantors or other obligors, or
security shall not affect the liability of Borrower, any guarantor and all
others now liable for all or any part of the obligations evidenced hereby. This
provision is a material inducement for Lender making the Loan to Borrower.

     8. OTHER GENERAL AGREEMENTS.

          8.1 The Loan is a business loan and is not being made for personal,
     family or household purposes.

          8.2 Time is of the essence hereof.

          8.3 This Note is governed and controlled as to validity, enforcement,
     interpretation, construction, effect and in all other respects by the
     statutes, laws and decisions of the State of Indiana. This Note may not be
     changed or amended orally but only by an instrument in writing signed by
     the party against whom enforcement of the change or amendment is sought.

          8.4 Lender shall not be construed for any purpose to be a partner,
     joint venturer, agent or associate of Borrower or of any lessee, operator,
     concessionaire or licensee of Borrower in the conduct of its business, and
     by the execution of this Note, Borrower agrees to indemnify, defend, and
     hold Lender harmless from and against any and all damages, costs, expenses
     and liability that may be incurred by Lender as a result of a claim that
     Lender is such partner, joint venturer, agent or associate.

          8.5 This Note has been made and delivered at Indianapolis, Indiana and
     all funds disbursed to or for the benefit of Borrower will be disbursed in
     Indianapolis, Indiana.

          8.6 The obligations and liabilities of Borrower under this Note shall
     be binding upon and enforceable against Borrower and its successors and
     assigns. This Note shall inure to the benefit of and may be enforced by
     Lender and its successors and assigns.

          8.7 If any provision of this Note is deemed to be invalid by reason of
     the operation of law, or by reason of the interpretation placed thereon by
     any administrative agency or any court, Borrower and Lender shall negotiate
     an equitable adjustment in the provisions of the same in order to effect,
     to the maximum extent permitted by law, the purpose of this and the
     validity and enforceability of the remaining provisions, or portions or
     applications thereof, shall not be affected thereby and shall remain in
     full force and effect.

          8.8 If the interest provisions herein or in any of the Loan Documents
     shall result, at any time during the Loan, in an effective rate of interest
     which, for any month, exceeds the limit of usury or other laws applicable
     to the Loan, all sums in excess of


                                       4

<PAGE>

     those lawfully collectible as interest of the period in question shall,
     without further agreement or notice between or by any party hereto, be
     applied upon principal immediately upon receipt of such monies by Lender,
     with the same force and effect as though the payer has specifically
     designated such extra sums to be so applied to principal and Lender had
     agreed to accept such extra payment(s) as a premium-free prepayment.
     Notwithstanding the foregoing, however, Lender may at any time and from
     time to time elect by notice in writing to Borrower to reduce or limit the
     collection to such sums which, when added to the said first-stated
     interest, shall not result in any payments toward principal in accordance
     with the requirements of the preceding sentence. In no event shall any
     agreed to or actual exaction as consideration for this Loan transcend the
     limits imposed or provided by the law applicable to this transaction or the
     makers hereof.

          8.9 Lender may at any time, upon prior written notice to Borrower,
     assign its rights in this Note and the Loan Documents, or any part thereof
     and transfer its rights in any or all of (i) the Collateral (as such term
     is defined in the U.S. Security Agreements), (ii) the Charged Portfolio (as
     such term is defined in the Charge Over Shares), and (iii) the Real
     Property, Chattels, Controlled Accounts, Policies, Intellectual Property,
     Investments, Rental Income, Contracts and Debts (as such terms are defined
     in the UK Debentures), and Lender thereafter shall be relieved from all
     liability with respect to such Collateral. In addition, Lender may at any
     time sell one or more participations in this Note. Borrower may not assign
     its interest in this Note, or any other agreement with Lender or any
     portion thereof, either voluntarily or by operation of law, without the
     prior written consent of Lender.

          8.10 Capitalized terms not defined herein shall have the same meaning
     given to them in the Loan Agreement.

     9. NOTICES. All notices required under this Note will be in writing and
will be transmitted in the manner and to the addresses or facsimile numbers
required by the Loan Agreement, or to such other addresses or facsimile numbers
as Lender and Borrower may specify from time to time in writing.

     10. WAIVER OF JURY TRIAL. THE LENDER AND THE BORROWER, AFTER CONSULTING OR
HAVING HAD THE OPPORTUNITY TO CONSULT WITH COUNSEL, EACH KNOWINGLY, VOLUNTARILY
AND INTENTIONALLY WAIVE IRREVOCABLY, THE RIGHT TO TRIAL BY JURY WITH RESPECT TO
ANY LEGAL PROCEEDING BASED HEREON, OR ARISING OUT OF, UNDER OR IN CONNECTION
WITH THIS NOTE OR ANY OF THE OTHER OBLIGATIONS, THE COLLATERAL, OR ANY OTHER
AGREEMENT EXECUTED OR CONTEMPLATED TO BE EXECUTED IN CONJUNCTION WITH THIS NOTE
OR THE LOAN DOCUMENTS, OR ANY COURSE OF CONDUCT OR COURSE OF DEALING IN WHICH
THE LENDER AND THE BORROWER ARE ADVERSE PARTIES. THIS PROVISION IS A MATERIAL
INDUCEMENT FOR THE LENDER GRANTING ANY FINANCIAL ACCOMMODATION TO THE BORROWER.

     11. CONSENT TO JURISDICTION. BORROWER (I) AGREES THAT ANY SUIT FOR THE
ENFORCEMENT OF THIS NOTE OR ANY OF THE OTHER LOAN DOCUMENTS MAY BE BROUGHT IN
THE COURTS OF THE STATE OF INDIANA


                                       5

<PAGE>

OR IN ANY FEDERAL COURT SITTING IN INDIANA; (II) CONSENTS TO THE NONEXCLUSIVE
JURISDICTION OF ANY SUCH COURT AND SERVICE OF PROCESS IN ANY SUCH SUIT BEING
MADE UPON BORROWER BY MAIL AT THE ADDRESS SPECIFIED IN THE LOAN AGREEMENT; AND
(III) HEREBY WAIVES ANY OBJECTION THAT IT MAY NOW OR IN THE FUTURE HAVE TO THE
VENUE OF ANY SUCH SUIT OR ANY SUCH COURT OR THAT SUCH SUIT IS BROUGHT IN AN
INCONVENIENT COURT.

     IN WITNESS WHEREOF, Borrower has executed and delivered this Note as of the
day and year first written above.

                                        BORROWER:

                                        CTI GROUP (HOLDINGS), INC.,
                                        a Delaware corporation


                                        By: /s/ John Birbeck
                                            ------------------------------------
                                            John Birbeck,
                                            Chief Executive Officer


Above Signatures Witnessed
by the Following Employee
of Lender:


/s/ Ryan T. Hendrickson
-------------------------------------
Ryan T. Hendrickson, Vice President


                                       6