Best way to get immediate relief is  as follows:

Put your palm (hand) near toe of your foot, then pull the foot (toe) towards your knee. If you don't get what I mean, then try to pull your toe of the foot of the leg crammed in such a way that the distance between foot fingers get closer to knee. I am not saying you should twist your foot. Just pull. This position is opposite to having your shoes in high heel shoes. If you stretch your foot like in high heel position, your not trying to heal from cram, but inviting another cram. Many people like to pose /stretch as if in high heel position, but it hurts more. Don't that in any case. When in as if using a high heel shoes position, note that the muscle in your calf Pidaula accumulates like a ball at one position, but when you pull your toe towards your knee, then you can realize that you are distributing your muscle to wider area. So, muscle cram happens because muscle tries to accumulate like a meet ball, and you should try to do something opposite to it: by pulling your toe towards your body. You can practice it as I outlined above. You will understand what I mean.

As soon as you feel that you are going to have a PIDAULA FARKANE (Calf? muscle on the back of leg between knee and ankle) muscle cram, then pull your toe towards your knee, you will get a great relief. At the same time, try to massage your muscle using another hand, so that you are relieving the muscle pain. It works. It is the only way, you can get relief. It happens when you are too weak, or walked too much ... or dehydrated. Drink enough water. If you have this kind of muscle cram in the night, then drink enough water before you go to bed. I am sorry, you have to wake up in the middle of night for urination. 

If you are a normal person, then it is better to check your body with doctor, you might be having stone in your body, and it can be side effect or symptome. I am not a medical doctor, but I have such muscle cram once in a while .... Particularly when I drink too much beer ..... or alcohol ... don't move at all for long duration e.g. don't go on walk for a week or so ... If it happens because you walk or run too much then it is because you are too weak.

Those with muscle cram problem, they should not go to swimming. It can be disastrous, particularly in deep side, so that you don't get drowned. The pain at the time of muscle cram is severe: almost in 8-10 point in a scale of pain designed my a Japanese.

All the best.