If the novelty phone works connected to the SH2 ( and therefore with Digital Voice ) it must be a ‘touch tone’ phone ( technically called DTMF) , for any number dialled to be recognised and responded to , otherwise the dialtone would continue after the dialed phone number had been entered , legacy PSTN lines , which presumably the telephone only line still is , recognises dual signaling, DTMF and LD ( loop disconnect ) , think of it as old rotary dial phones, if the novelty phine has a switch DTMF/LD , try the novelty phone in the ordinary line but with the switch set to LD .
If the PSTN equipment in the exchange ( which is pretty elderly, one of the reasons it’s being retired in favour of IP telephony like DV ) has a fault it may not be responding to the DTMF ‘tones’ the novelty phone is providing, but may still respond to LD , but to prove this conclusively you need to connect another more modern phone to the phoneline and see if this alternative /replacement phone cannot make a call either …if it can , the novelty phone is the culprit and needs replacing, irrespective of it being usable on the DV /SH2 ‘line’ , if a replacement modern phone doesn’t work either ( on the telephone only line ) report the PSTN line as faulty.
This is only relevant with outgoing calls , the signalling, DTMF or loop disconnect doesnt affect incoming calls , you don’t sate if it’s both incoming and outgoing calls that you are having an issue with .
