Use a butt connector. Crimp, then solder.
Many crimp style butt connectors are made of fairly soft materials and do not make a very solid mechanical connection. The standard "buy them by the box, red/blue/yellow" butt splices have practically no ability to provide for a rigid mechanical connection. The connector body may be tin plated copper that is soft enough for someone to form a crimp with a hand-tool.
There are more durable mechanical crimp connections like Brundy connectors that use a hydraulic press to "swage" the metals together (deformation of the metals into a plastic state so they really are one piece). You can see some of those used on high voltage transmission towers where there may be tens of thousands of pounds of tensile force on the connector.
Most people are not going to have the means to install a hydraulically crimped connector as they are sized very exactly for the wire in question. I would not trust a standard, off-the-shelf tin coated copper compression crimp for anything other than a temporary, emergency repair.
The Bell/ Western Union splice is very robust and reliable, even if you do not solder it. Remember that solder is not to make a mechanical connection. In this application it is to provide a filler material on the electrical connection.