Transmit phone line signal through the house using powerline-ethernet adapters

You cannot do this. I will not go into the technical details of why this is impossible, given that Mokubai ♦'s answer explains this very well.

I still felt it necessary to write an answer, because this is a classic XY problem. You have a problem, came up with a solution, need help with that solution and ask here, but your solution is not the best way to solve the problem.

I identified your problem as follows: You want a WIFI or ethernet access at a place across your house but your internet is not entering at that location. (this is a common problem actually)

The obvious solution here (this is what everyone does) is place the ADSL modem/router where the internet comes in, and get the data (ethernet) signal across the house from there. This can be done using a Powerline adapter.

It would not be the best and most reliable option though. An ethernet cable is definitely the best option. A wifi repeater could also work but it will reduce the speeds. A PowerLine Adapter will also work, but is known to require a reset every once in a while.

Lets assume you go for the Powerline adapters option, given that that was included in your question and you have them, you would need something at the other end (in your livingroom/bedroom/etc) to be able to give one or more devices access to the internet. If you only use one or more pc's, you can use ethernet cables to with one or more powerline adapters to create your network. If you want wifi support, you will need a wifi capable broadcaster, such as an access point or a wifi-router. A wifi router would allow 3 ethernet ports and wifi access in your room with just one set of powerline adapters and give you great expandability. I mention 3 ethernet ports, not 4, because I would plugin the ethernet cable in one of the LAN ports, not the WAN port. This will ensure that the modem/router creates the whole network, and you won't have to make port forwardings twice.

Powerline adaptors take data, as in ethernet packets, and convert them to an analogue/digital signal that can be transmitted over electrical wiring.

Your phone line ADSL works in the opposite direction. It is an analogue signal carrier encapsulating a digital signal that needs your modem to decode.

You are effectively wanting the powerline adaptor to do analogue-to-analogue conversion rather than digital to analogue. They are not analogue to analogue converters. This will not work.

Telephone extension cables are cheap. There are telephone over powerline adaptors, but I would expect them to be voice-only. ADSL uses a lot of high frequency signals that will probably be lost by any such adaptor.

Get an extension cable in order to move your telephone point.

The other problem would be that you would be effectively putting your phone signal onto the local power grid. While some power filtering may happen at the power inlet to your home it may be effectively a direct connection and could allow anyone with a similar device to effectively steal or otherwise mess with your internet connection.

To explain a little more VDSL (and ADSL) uses a complex set of transmitters and receivers working together. They effectively inject multiple signals on to a data line to achieve what is a large number of relatively small individual bandwidth signals. Any device to retransmit these signals would have to understand the types of signal expected, the frequencies and magnitudes and so on as they need to deconstruct and reconstruct them.

Powerline adaptors might function in a similar way, but due to them being proprietary and only intended to be used with another identical device, are under no constraints to be interoperable with major telephone devices. They don't care about ADSL or VDSL or any other transmission scheme except their own. As such they tend to make up their own scheme and could well not be sensitive enough or have enough bandwidth at specific frequencies to be compatible even if they accepted analogue signals on both sides.

Powerline adaptors are also (generally) actual intelligent network devices. They don't convert the electrical signals underlying the Ethernet protocol to "audio on the line" but instead accept Ethernet data packets and convert those instead. Otherwise there would be a lot of unnecessary noise on your powerline, even when nothing was being transmitted between devices on your network. Think of Powerline devices as similar to Wifi devices, except that their signals get pushed onto (coupled onto) your electrical wiring.

As a result the only device guaranteed to be able to convert ADSL to powerline would be an ADSL-to-powerline adaptor and those don't appear to be that common a thing, probably due to there being a lot of standards requiring support to ensure compatibility.

Most solutions I can find are along the lines of the answer posted by LPChip: use a proper ADSL modem where you can, then an Ethernet-to-Powerline adaptor or other solutions.