Install your preferred GNU/Linux distribution on an old PC, then install Asterisk on the PC using your distribution’s package manager eg. apt-get install asterisk on Debian GNU/Linux, then check out /etc/asterisk/ directory for configuration, then try to get some used SIP desk phones and/or SIP soft phones to connect to Asterisk and start making test calls.
I am not sure what is available in your area, but a couple of gently used SIP desk phones and an old x64 PC might be reasonably priced hardware options for you to consider.
A ‘new’ install of Linphone is pretty hard to beat price-wise.
Many years ago I picked up a bunch of used Polycom IP 500’s off Ebay for $20 a pop. Still my primary desk set.
I would rate a used ‘quality’ phone over a new ‘cheap’ phone for ‘survivability’ and access to repair parts.
Looking now, I see ‘lot’ prices for IP 500s and IP 550s for $5 to $10 each. I didn’t check to see if that includes power supplies – which are kind of important
To add on what’s already been mentioned here are a few training videos that you might find useful. Note they are a couple of years old now, but most stuff will still apply: