99% of all services out there have v.92... you can take advantage of v.92 Modem on Hold capability. All that is required on your end is a v92 modem (which all new modems are) and call waiting. If you have that, and you're on the internet, and somebody calls you, your modem will listen for the call waiting beep, and notify you on your screen that you have an incoming call (and if you have caller ID, who's calling you). You can either choose to accept the call or ignore it. If you choose to accept the call, your internet service is put on hold while you take the call (different ISP will have different limits on this... they usually range from 2-6mins though), and when you're done, your internet is resumed.
There's no fee for this. You just need a v92 modem and call waiting.