[Telephone] Use Charter phone & KEEP one tradtional phone line?

Have my house wired in walls properly for two phone lines. Currently with local POTS copper wire service with Century-Link (used to be Qwest). They charge me an arm and a leg, but are super reliable. Been tempted to go with Charter Triple play. Could save big bucks. Want to have charter on line-1 house wiring and regular phone company on line2. (Having a basic service on on traditional copper wire with no features costs same as a second Charter line, but runs when the power is down or if Charter has some rare major system problem). 5 years ago I set up a work order to do that but when the Charter tech got here he said he could not put the charter line into the same house wiring as my existing other line service. That I'd have to use charter's phone line-wiring on a cordless base with handsets.... that he couldn't put it into my line-1 wall wiring while Qwest had their POTS phone line wired into my line-2 wall wiring. Made no sense to me. But I had to cancel and send him away. (Took major jumping through hoops to get my phone number ported back to my local phone company). So I was left with a bad taste on that attempt. Today spoke to two separate Charter reps who both seemed knowledgable, and both assured me they could indeed do what I wanted and had done so many times. Only had to have the work order specify that and say "single port" or something like that. My question is has anyone here done that (had Charter phone on one line, other provider on other, and both wired into the house wall jacks) or can speak knowledgably on that topic? Would love to do it, but nervous about having it bollixed again. Other question: Charter's little built-in battery backup which they sell for $40 they claim 8 hour standby and 5 hours talk. Seems like a lot for that little battery... maybe exagerrated claim? Seems like I'd be a lot better off buying for $60 a much higher capacity UPS. Any thoughts? For that matter, any, good, bad, or interesting reports on experiences with Charter phone, worth hearing. FWIW: I'm in the Medford Oregon area. All replies appreciated. -- Alex C.