PBX outdated, nowadays, use voip or ip phone like > https://www.yeastar.com/

I had set up for 20+ before, and ip pbx makes more sense. a budgetary price is:

1. 1 unit of A420 IP-PBX (4 FXO, 100 Extensions capable) = RM 1988
2. 22 units of ip phone = RM 180 x 22 = RM 4000 (3 already provided)
3. 1 unit TP-LINK TL-SL1226 24 + 2G Gigabit POE Switch 24-port = RM 510

u need 2 to 4 lines from telekom for inbound
u will use a voip provider/gateway for outbound via internet (approx RM 100 a month only, 1100 Min free, after 9 sen / min for call out, and cheaper overseas calls)
u will need LAN cable (if PC LAN cable already got already, they can share) else substantial cost here ...
u need consider maintenance and installation fee, minimum amounts,

So juz budget 6 to 7 K ... total.
It has been mostly hassle free, but beware hackers, coz network/internet based.

https://youtu.be/6eVEI_ORfmw

RJ 11 - needs a converter to IP, but less reliable.
is either ip or analogue, hard to mix.

but analogue system, you cant print reports and see logs of abusive users.

no need server, juz buy appliance.