Many VOIP devices with FXS ports do not support traditional rotary dial phones.
One possible solution (untested) is a Dial pulse to DTMF (Touchtone) converter between the rotary dial phone and the VOIP FXS port

Example:

• Dial Pulse Phone <-----> Pulse to DTMF converter <-----> VOIP FXS port on a SIP ATA

Some possible choices:

• Oldphoneworks VOIP Pulse to Tone Converter (LPT-310) http://www.oldphoneworks.com/antique-phone-parts/parts-and-pieces/pulse-to-tone-converters/ $49.95
• Rotatone Pulse to Tone Converter http://www.rotatone.com $39.95
• Mitel Smart-1 dialer (requires programming)
  • Source: http://www.dialerbuddy.com/mitel.htm Approx $36
• Sandman PULSE TO TONE CONVERTER (LPT-310)
  • Source: http://www.sandman.com/digit.html Approx $45
• Panasonic KX-TA624 hybrid key system
  • Factory-default configuration will convert pulse dialing on an extension to DTMF on an outgoing CO port. Unit supports 10 or 20 pps dials.
  • Note: Early model KX-TA624 units tend to spontaneously reboot when ringing old-style electromechanical ringers. This seems to be related to inductive spikes rather than power consumption. The late-model KX-TA624-5 does not have this problem.

Known to work, tested:

• Dialgizmo Pulse to Tone Converter - Approx $40
  • Will convert rotary pulse dialing to DTMF tone. Allows dialing of * and #, speed dial and LNR
  • Works well with the lower line voltages found on some ATA FXS ports
  • Source: http://www.dialgizmo.com
• Oldphoneworks LPT-310 Pulse to Tone Converter - $49.95
  • Great unit. Works flawlessly on most VOIP services! Will not register double-digits like some older models.
  • source: http://www.oldphoneworks.com/antique-phone-parts/parts-and-pieces/pulse-to-tone-converters/
• Rotatone Pulse to Tone Converter - $39.95
  • Designed for installation within a Rotary dial telephone. Some technical expertise required for installation. Adds *, #, Speed Dial, Last Number Redial and Hotline functionality to a rotary phone. Installation services available. Swedish and New Zealand versions available. http://www.rotatone.com