Ed_Gee wrote:
Here is one possible explanation: Those cordless phone are often duplex operations with one side of the operation going out over 900 MHz or 2.4GHz, or 5GHz...and they receive on low frequencies sent over house AC wiring. When you are running your system with 12VDC there is no AC wiring available for the low frequency antenna.
If you are referring to a standard cordless phone I'd like to know the make and model of any phone in the last 20 years with any operation below 72MHZ. Any digital phone these days is spread spectrum or a channel hopper at the least, and duplex operation is packet shared on exactly the same frequencies.
OP- have you watched the off hook DC voltage as well as static? All signaling and audio should be transferred over the RJ-11. If the design is such that the electronics grounds need to be DC isolated and the manfacturer is relying on xformer isolation of the wall worts... That sux.. You can find DC-DC converters on eBay that might work but it sounds like you need to do a little research with a DVM or scope and see if ring ground is being carried through via DC ground and giving the appearance of the phone hung up :(