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 Sipura 2000

Some possible choices:

• TelSoft VOIP Pulse To Tone Adapter (Plug and Play)
  • Source: http://telsoftdirect.com/index.asp?PageAction=VIEWPROD&ProdID=19 Approx $54
• Mitel Smart-1 dialer (requires programming)
  • Source: http://www.dialerbuddy.com/mitel.htm Approx $36
• Sandman PULSE TO TONE CONVERTER
  • 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:

• LPT-300 - Pulse to Tone converter - http://www.dialerbuddy.com/lpt300.htm Approx $50
  • Thoughts on this: This is a simple unit. Do not put this in line where the upstream carrier supports both pulse and dtmf, as that will register double-digits dialed. It works flawlessly otherwise.