If you sold fibre for a living thats what you would say too, then when it doesnt work, just say oh dear your system is one of the 1%.
Of course your installer has told you it needs to be upgraded, you asked them their advice, and that is their advice. Most here would give the same or similar advice, not because anyone wants your money, we make no financial gain giving you advice but you came here asking for advice.
Your cheaper alternate would be to add a gsm dialler as suggested by other, this will have additional monthly fees for SIM etc, there are wifi signalling units out there too, but again there will be costs for these too as they work via servers which cost money to keep running.
What I will say is your Vocom will be no good after 2025 as openreach plan to digitise all pstn by then anyway.
My advice would be upgrade the panel if you can afford to, if you cannot, the choices are GSM or Wifi if your installer has a universal Wifi solution