It can be easily done since most voip client software registers itself capable to handle SIP URI. This means that you can add any sip link to your webpage and it will be recognized by browser (will launch the pre-installed client if somebody clicks on your link). This is like a normal http link but with sip. Example:
sip:\\phonenumber@voipserver.com.
Let's see if it is working from here using my voip server:
click to call
(it works if your softphone will launch a call to testivr3 if you click on the above link)