Not as far as I know.
However, using a device such as the Gigaset N300-IP would enable you to use several SIP services plus a single landline (e.g. PlusNet) seamlessly through the same DECT handsets, although obviously the landline could only be used where it is physically located as opposed to the SIP services which can be used anywhere there is access to the Internet.
Re: Can I use plusnet telephony with a SIP phone?
Does plusnet support SIP and if yes: where can I find the SIP settings (server, SIP ID, password, etc.)?
To confirm what @RobPN says, they don't. Some SIP providers ( localphone is one ) will allow you to register your landline number as an outgoing CLI. So using the N300 as suggested, would allow outgoing SIP calls to appear as coming from your landline number. Obviously incoming calls to the number would still be routed to the landline.