Can I use People+content to send content to an RMX meeting room to which I'm connected using my phone only ?
I'm puzzled because in People+content I can only specifiy a meeting password but not specify a meeting room number
no, not possible. The P+C IP does only connect to an endpoint as content source - it´s not a "video endpoint" itself.
So, in your case you would have to connect your notebook via p+c ip to a hdx or rpg -> then call from the hdx or rpg to the rmx meeting room -> then send content.