I think what you would like to accomplish will require the use of IVM and NCH's Free Axon software.
Basically if I understand you correctly, you would like the caller to be able to phone in and select from a series of options for what he or she would like to do. And depending what the user selects, he or she is able to either leave a message or ring the phone in the office for a person to answer. Call screening is also supported by IVM when leaving voicemails so you can hear what is being said at the time the message is left (Although I don't recommend this in a doctor's office as someone may be discussing private information over the phone and that shouldn't be publicly broadcasted). In any case it's easy to setup IVM for listening to messages in different mailboxes and also having the option of sending them via email or collecting them remotely by internet or by phone if you want.
Now it all depends on how you plan on interfacing with the phone system you already have. Here are your options:
1) Use a voice modem
Pros: You can use your existing phone system / number, simply plug and play, install modem driver and you're ready to start receiving calls.
Cons: voice modems don't have excellent quality. Voice modems can't ring the phone connected to the computer.
2) Use an FXO Adapter
Pros: You can use your existing phone system / number. Works with Axon and IVM to deliver a simple yet advanced phone management system. Can ring phones connected to the axon pbx software, this can include free softphones (answered with headset connected to computer), IP or USB phones
Cons: Cost is higher than voice modem, also added cost of hardware phones if you do not want a softphone
3) Move to VoIP instead of your existing phone line (what most businesses are moving towards)
Pros: Depending on your provider you may be able to accept more than one incoming call at a time. Your SIP VoIP account will work flawlessly with IVM and Axon. High quality calls, delivered over your internet connection. Does not require FXO adapter, therefore cut cost. Can do everything you need. Cheaper call rates. Supports multiple phones connected to system, each with own extension (for calling internally) and can be grouped so for example all phones ring when call is external.
Cons: requires reliable high-speed internet connection. Added cost of hardware phones if you do not want a softphone. You may be required to get a new phone number for your office, the good thing is most telephone companies offer a forwarding feature to forward your calls to your new VoIP number as to not disrupt your office. If you decide to keep your existing line as well as have the VoiIP line, you are paying for two monthly billed phone services.