DECT Content with Samrt Phone

Hello everyone

we have o2 DSL at home with an o2 home Box 6741 router.Connected to this is a Siemens landline telephone.

We would like to have a smartphone app instead of this phone, so that the smartphone serves as a landline phone when the smartphone is in wireless range.

I have seen, DECT 1 & 1 and the Fritzbox itself offer this app already.

Can you get this app from o2 ??

Thanks