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With TWC, not sure about Charter, the phone modem is free of cost (any cost is included with the monthly charges). There was no charge for installation to me probably because I was moving to a triple play package. They even ported my old number (very convenient) from AT&T.
hopefully you have good enough levels for an additional 3.5 dB loss splitter, you use the twc one for phone and yours for data. If your US is higher than +48 dBmv or the DS is below -5 dBmv, another splitter won't work
While there's a lot of 6141's still in service, I'd suggest a 6183 as it will handle the extended bandwidth TWC's been pushing on us.