Answering a cell phone call using Uniden D1780-BT

I have an iPhone 6 paired with my Uniden D1780-bt answering machine.  I am able to use one of the portable handsets to dial a call then press the cell button and have the call go through using my cell phone BUT when I receive a call both the answering machine and the cell phone ring but I am unable to answer the call using one of the 4 handsets--just able to answer using my cell. I do not have a land line.

Any advice?