Because of network coverage differences, I've had to use Sprint too. Cingular's customer service is certainly better than Sprint's. (Don't get me started on Sprint's store staff in DC.)

What I do now is use a Vonage-hosted phone number as my cellphone number and forward it to either of my cell phones (or elsewhere). It lowered the number of minutes I use and avoids my need of checking on my other cell phone's voicemail when I get lazy or something.