USB adapter to use softphone

I have an application where the internet will come in via a USB plug.
That will connect to a laptop running windows XP.
I want to run a SIP softphone on the PC - e.g. x-lite.
X-lite is good because it is possible to set a dial plan.
With all that in place I want to be able to use an adapter, connecting
via USB to the laptop, which will allow connection of a standard
cordless phone. Additionally, a connection to the PSTN line (which
may, or may not exist) is desirable, but not essential, with,
preferably, access via the telephone keypad.
It must be possible to dial out using the phone keypad.
I have seen a number of products specifically for Skype (not what I am
looking for) , and odd ones supposedly for SIP soft clients but I am
looking for recommendations - Has anyone tried any of these or know of
any which will fit the spec.?
I am looking for a cheap solution - e.g. no more than about £20.


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