My last cell phone could have an external antennae attached to it that did just what you want. It cost about $20. Unfortunately, the manufacturer of my new cell phone thinks the phone is so good there is no need for such an option.

David Pogue wrote about a new service, T-mobile Hotspot @ home. For an extra $10/month, you get a wireless router and a compatible phone. When you are at home, all of your cell calls go over VoIP and so you don't use up minutes. Also, you will get good reception through your home. The savings form being able to buy a plan with fewer minutes should cover the cost of the extra plan. Also, when you are at any wifi hotspot -- including places overseas -- you can make free calls.

Here's the link: http://www.nytimes.com/2007/07/05/technology/circuits/05pogue.html?ex=1185076800&en=973c4ce90add3ad6&ei=5070