You could upgrade the tda to a tde with main board and dsp card then use ip dect cell stations

plan b would be to use 4 Cisco SPA3102 on each side. they can take a slt extension in on side a and return it on side b. lift off handset on side b and it gets dial tone from system. have used before as a cheaper solution.

you can put cordless phones on side b depending on the area you want to cover

“I have not failed. I've just found 10,000 ways that won't work.”