To explain: All of the cordless phones I have owned that have used proprietary batteries have had--at most--45 minutes of battery life when the batteries were new. With the most recent version of such a phone, within a month of getting new batteries, the useable battery life was down to ten minutes. Far less, if you happened to forget to put the phone back in the charger for any length of time at all. The replacement batteries costs around $20-25 (x 4 handsets--so $100 for about two to three weeks worth of reasonable call length). By contrast, the Netgear/Skype phone I own--which uses two standard AAA rechargeable batteries that costs around $7 to replace (for the pair)--has never gone dead in use--ever. Even if left off the charging cradle for days on end, one can still use the handset for several hours. Ditto for the Verizon phones I just purchased which also use standard off the shelf batteries.
My advice: forget about trying to make money off of batteries. You won't make any as soon as generic suppliers undercut whatever margin you have. You'll do nothing but piss your customers off. Which means you lose customers (and money) in the long run.
https://fjallfoss.fcc.gov/prod/oet/form ... or_pdf=pdf
However, this may be pre-production and could change for release.