So, this morning I called 1&1 to ask whether porting my DSL connection is as simple as telling German Telecom to transfer my phone number downstairs, and the lady said no -- that the DSL connection is separate from the phone connection.
In fact, they can't even just switch the connection downstairs; rather, they have to deallocate the port (at the DSLAM, I presume) and allocate a new one. (A colleague of mine said that that might be a problem if ports are scarce and the deallocation results in someone on a waitlist grabbing it, leaving me to end up on a waitlist myself.)
At any rate, that'd cost €99 (which the Telecom company charges 1&1, so they pass it on) and might take four to six weeks. If, after the old phone connection is cancelled, I call 1&1 after five days or so, they could try to request a whole new port, which might speed up the procedure and take only three weeks total.
I suppose technically I could connect to the Internet using ISDN (which should, after all, require only the working phone connection) or -- if I bought myself an analogue modem -- with an analogue connection through our analogue-to-ISDN converter which our phones are connected to... but that would be metred Internet again, and Stella nixed that idea. She suggested I use the opportunity to read some books I had always wanted to get around to.
It's not even the low-bandwidth thing that's the biggest problem, but the pay-by-the-minute thing.
And another annoying thing is that Stella asked me this morning whether I hadn't wanted to move to a DSL connection hosted by German Telecom if we moved somewhere -- which I had considered. Only, since I thought at the beginning of the month that we'd stay here for the appreciable future (since the place in Heimfeld never rang us back), I switched from a 5 GB volume tariff to a flat-rate connection, which extends my contract by a minimum of 12 months. So even if I switched IP connectivity providers, I'd still have to pay the service charge for changing my contract and have to pay twelve monthly rates, which makes that prospect unattractive.
(But who knows? Maybe magically everything will work out fairly quickly.)