The various services had different peicing tiers. 3615 was the most used and would cost a few francs per minutes. Some sites would force you to spend some time

The various services had different peicing tiers. 3615 was the most used and would cost a few francs per minutes. Some sites would force you to spend some time on their 3615 number so you could accumulate 'minutes' of use through the cheaper '3614' number. They used that scheme to modulate the cost of their service.