Why won't my new(ish) telephone ring?

I was recently gifted a rotary telephone. I'm not sure when it was made or what model it is, but I believe it is a 300-series GPO telephone manafactured between 1937-1959.

When I plugged it into my phone system however, I could not make it ring. I could call it, then pick up, which would connect me, but it wouldn't ring. This, as you can imagine, does not make a very helpful telephone.

I set off to fix this; to make my phone useful to me.

The first thing I did was disassemble the unit - to see if there were any loose connections to fix.

This is the inside of the unit:

And this is where all the connections are made.

As you can see, all the wires were connected, and the connections were secure. I used a multimeter to check if any ring voltage was making it's way to the ring solenoids, but this only seemed to be around 3 VAC. That wasn't right at all.

After doing some more digging, I found this forum exchange: https://community.bt.com/t5/Home-phone-including-Digital/Rotary-Phone-not-ringing/m-p/2286317.

This seemed to suggest that just putting a microfilter in the connection would somehow make the phone ring. I didn't believe that could work, but gave it a go anyway:

And the result:

Hurrah! A working telephone ringer.

It cannot yet dial out due to my SPA-122 not supporting pulse dialling, but I have ordered an adapter to get that working which should arrive soon!