[Xylakant]

I actually have my own Cisco Phone adapter to connect my old analog phone/answering machine. I'd assume that a true SIP phone would work as well. What's questionable is whether I could authenticate from a different network than my home network.

I know, however, that both Vodafone and Telekom in Germany offer products that allow connecting VOIP phones from anywhere to a virtual phone appliance.

[Tijdreiziger]

That's interesting. In the Netherlands, all the telcos (with the notable exception of XS4ALL) lock down the modems and don't give you the settings and logins to use your own equipment (although the Consumer and Market Authority is still researching whether this is actually legal).

[Xylakant]

Locking the connection to a provided modem is not legal in Germany since 2016-08-01 by law (FTEG, replaced by FuAG in 2017) The provider is obliged to hand you the required credentials to connect your own hardware, though getting the actual required configuration may require some puzzle work, but most providers give config examples for at least the most common hardware. My setup for example consists of a draytek modem with unifi network hardware and above mentioned Cisco phone adapter. I don’t even have a piece of Telekom-provided hardware here. Took a bit of googling to get IPTv working, but other than that it’s a perfectly stable and capable setup. Many people use Fritz boxes which are a bit less capable but easier to set up.