I have some corded telephones that uses a landline, and I wana try to make them ring (without the landline), and also try to make a intercom out of them. So, do you have any ideas for this?

I don't remember what it is called, but there is a device that you can plug a pair of phones into.  When one phone is picked up, it causes the other phone to ring, and when both are picked up, they can talk to each other.

Do you need the ringing to function, or just the voice?
And the telephones in question are some old types (with just a carbon microphone, transformer, few resistors and capacitors and a headphone speaker, so without any electronic) or some more modern electronic one (with active electronic amplifier and direction splitter bridge)?

When no signalling needed, the old phones are simple: Just connect them in series onto some 12V (or so) battery. Some electronic ones will work in that way too, but not guaranteed.
A bit more complex is connection of each phone via two ~2kOhm resistors to about 24VDC (to provide a DC bias supply), with ~1uF capacitors between these two (to couple the voice signal).

A bit more circuitry (two SPDT 24V relais, 4 diodes, one 24VAC transformer, one ~10kOhm resistor, one 220uF/63V and one 2,2uF/100V electrolytic capacitors) will make quite simple box allowing even some simple signalling (if you pick one, the other rings, if you pick the second, the voice connection is made, if you hang both the thing is reset back)

Both, ringing and voice functions.

https://www.youtube.com/watch?v=AxXsIQDafog   you can skip to the 6:30 mark if you want

Would this circuit work on electronic phones, like the Panasonic, AT&T, and V-Tech ones? Plus do you need a bridge rectifier for the circuit? Because electrolytic capacitors can explode on AC.

Depends how they behave from DC bias perspective. The key is, both phones, when parallel, get about a correct DC bias current. If the phone is designed so it forces constant voltage, the one with this voltage slightly lower will take all the bias current and there will be nothing left for the second one.
But you may try. Maybe some series resistor (~100..300Ohm bypassed by a 10uF capacitor for AC) with each phone may provide enough ballasting to ensure a reasonable bias distribution.
This is the reason both phones have to be of the same model - so their clamping voltage is about the same.

The old non electronic ones have the benefit they are behaving as a true resistor, so first they distributre the bias current inherently well, plus they are rather imune if the current is off (just the volume changes a bit)



They are not supplied by AC, but by DC. There is a rectifier, just halfwave, to allow the common AC winding to supply both DC for voice circuit bias, as well as AC for ringing.
The 100uF is a filter of a half wave rectifier (the left diode), generating about 30VDC for the system supply.
The 2.2uF ringer coupling is biased by the auxiliary diode from the 30V DC line, via the 10kOhm resistor. Normally there is no DC path from the capacitor to GND: Either the phone is disconnected by the relay, or the ringer is AC coupled within the phone itself, so again no DC path, soi the 2.2uF remains charged to about 30..80V (the 80V takes into account the OCV of the unloaded transformer secondary during idle).
Only when the called phone picks up, there is for a brief moment DC path to GND. But this means first a zero DC bias for the capacitor (so still marginally OK) and second the presence of the DC path energizes the relay, which then disconnects the ringing circuit, so allowing the 10kOhm resistor + its diode to charge the capacitor back to the 60V.
So only for some 20ms or so the voltage may reverse a bit. Not seen this as any problem yet, the real problem is the slow electrolysis and that needs time and no recovery (here the ever present 60..80V will ensure decent maintenance of the oxide layer)
To make sure there is really no reverse voltage across this ringer coupling capacitor, you may increase its value (even 20,,30uF are OK), you should still maintain its voltage rating at least 100V.

OK, do you have a video of this circuit in action?

Some cordless phones with multiple handsets (like Panasonic, VTech or Uniden) can be used as intercoms without a phone line.

No, I don't have it anymore (it got scrapped when I moved from my parents more than 20 years ago and then the cameras were not that spread as they are today)...

But I think it is simple enough for anyone to be tried out, even just for the fun of it...
The transformer could be virtually anything small (here these signal light transformers were dirt common in any "pre-computer" automation control unit, I doubt something similar was not common elsewhere for that purpose, even with better insulated primary connection), the thing draws barely 50mA when ringing and barely 30mA when connected.
The relais i used were "true" gravity telephone relay from 50's which didnt respond on AC so didnt required the series diodes, but with the diodes in series any 24V relay will work (the diodes, together with the ringer coupling capacitor prebias do separate the AC ringing voltage from activating the relay). So with these old relais the diodes in series with the coils, nor the 10kOhm resistor with its diode were needed. But these components will never cause any harm and cost cents, so today I would put them there anyway, to be sure the thing won't do any glitches.

I also was looking at ebay, and they have the four line version of the telephone line simulator by Teltone, and its pretty expensive.