How can one phone page or talk through another phones speaker

There are a couple of options - off the top of my head you can make a shortcode or button with the "Call Direct" feature that does what we used to refer to as "Voice Call" on Nortel. I believe there's at least one other feature that does the exact same thing called "Automatic Intercom" or something like that.

I always created 2 Short codes.

#2XX
Voicemail Transfer
"#"2N

*2XX
Dial Direct
N

Obviously those are for 3 digit extensions that begin with a 2...but you get the gist of it.
That allowed me to have a manual for customer that said, press TRANSFER, POUND 201, to transfer directly to voicemail...

And now user can dal STAR ext number, or STAR and BLF button, to go direct to intercom.