SQL Ledger can easily be integrated with the German VoIP-service sipgate.de - sipgate.de provides a plugin for the Mozilla Firefox webbrowser to link phone numbers that are displayed on a website into links which in turn trigger a softphone application to call that particular number.
Sipgate's accounts are paid for on a prepaid basis. The money you pay to sipgate.de will be credited to your account and you can then make phone calls at fairly low rates. So you save money and don't waste your time dialling on your keypad.
A possible use case might be that one day you are checking unpaid invoices and decide to call up the customer to find out of the whereabouts of your money. The call can then be initiated right through SQL Ledger.
You can get the plugin from the following URL: https://addons.mozilla.org/de/firefox/addon/sipgateffx/
Sipgate's offerings can be found at: http://www.sipgate.de