How to make a local conference call on IP phones?

Question

Telephony

In general, someone advised my boss Gigaset C530A IP phones
And he said that at events he uses instead of walkie-talkies, the type called, a headset in his ear and that's it.
I searched everything and I can't find how to do it, the only options I found are either to buy a SIP account and pay 2k per month, or to make a virtual ATC myself.
The only thing that turns out is to call from handset to handset, as well as make a general call to all handsets, but with a general call, when one answers, the rest of the call is dropped.

Answer 1

An IP phone is also a phone to connect it with a PBX. It has a conference call function, but for this it must have an active number: external or internal, otherwise how will it merge calls if the callers have no numbers, nothing. And so everything is in the instructions.