Ask HN: Why no mobile phone can also be used as a DECT phone?

I just wonder since a long time, why no mobile phone also can be used as a DECT phone. Wouldn't it be nice to connect with your companys mobile phone to your companys DECT phone network. Now I have to carry my companys DECT phone and the companys mobile phone around. It can't be that hard to do it. Also the hardware is from different companys, so it's not "we can sell 2 phones". So why nobody does it?