When it comes to cable splices, variety is to be avoided. Better yet, avoid splices altogether. But when splices are unavoidable, using the same cable/wiring type throughout minimizes consequences.
Depending on the type of signal being conducted, the difference in impedance on either side of the spice can result in reflections and significant signal degradation.
Although the signaling demand of this telephone application is rather low, the wire nuts could have been avoided, and a much cleaner installation achieved, had they “punched down” a length of 24ga wire instead of glomming on a piece of speaker wire.