Sometimes the TDxxxx VoIP phone system dials before the Dial tone is ready. This causes the first digit to be lost and the call to fail. Usually you get a message from the phone company telling you to “dial 1 before the number” . To fix this add a “w+” to the dialing prefix in the Zap trunk settings.  Go to  the ‘Trunks” link under the ‘PBX’ tab and select all the ZAP trunks configured on your system. This forces the hardware to wait a half-second (for each “w” you put) for the dial tone before dialing digits.