A free sending works like this:
You sacrifice any amount of points to it that you like. The amount that you get back depends on how quickly you call them, once you have spent the points. Add 1 for every solstice or equinox that the sending remains uncalled, to a maximum of 4. Divide the number by 5. Multiply this number by the amount sacrificed. This represents the amount returned. The force of the sending's attack is however much was sacrificed to them.
E.g. If you sacrifice 100 points of power to the sending, and use them after a solstice and equinox have passed, you will get(2/5ths of 100 = ) 40 points back. If you wait for a year and a day, you will get (4/5ths of 100 =) 80 points back. The sending's attack is considered to have a force of 100.
The site of the sending is in different places for different people. If you find it when you were heading down to the shops to buy a packet of cigarettes, then that's where it is for you. If you find it in the 3rd door from the bottom of the Abyss, guarded by a horde of slavering horrors led by Verjigorm, then that's where you have to go to find it again.
Once you have called them they become uncallable by you until you visit them again, wherever you know them to be. They manifest at different rates largely depending on how much they like you.
Your other characters may know it to be in different places, and that's true for each of them. You may not benefit from the knowledge of your other characters with respect to free sendings.