There is a complete VoIP door intercom unit available that is SIP compatible and works with asterisk, as long as you set up astersik to register the unit without password.
Manufacturer is ITS-Telecom, israel
Product is Pantel or Pancode depending on model type
address: newbielink:http://www.its-tel.com/info.asp?id=48 [nonactive]
I import and sell the units in sweden. If you can locate a local supplier...great
It works on POE as well as 12Vdc but you need a separate 12vdc power supply to open your electric lock.
The caller presses a call button, your phone rings, you talk 2-way, decide to open the door by pressing button 8 on your dtmf phone. (programmable)
The unit should be available with a recess in the metal housing to take an analog camera which you would then connect to a small web camera server...this way you have exactly what you want.
Best of luck