So you are looking to "push" an IP phone the the customer, and use the web as your transport? Interesting idea, but I see a couple of problems with that:
1) I believe the softphone is a client based app, so I do not think you can load it as a dynamic install to a customers PC, then have it removed once the call is complete, all across the web.
2) The fact the web connection they are using will never be consistent, you have no way to control the QoS of the call.
3) The customer would have to have a headset or hard phone or a microphone and speakers connected to their PC. I know this is rather common, but it would have to be required for every customer and assumed.
4) Just the amount of time it would take to get this to work. Even if you can get the softphone to install, the time required to set this up could be a deterrant. You could do it for every one of your customers, but then you are buying licenses for every one in your customer base; could be very expensive.
Some of this is just my $.02 since I do not know your customer base, but I felt it was worth mentioning.
Good luck,
Scott M.