<<<<<I want to know that is it valid and permissible to make a confession over the phone with your priest or confessor who knows you personally? >>>>

No, this is not normally permissible because Confession is meant to be a personal Sacrament. That is, it is meant to be performed in person, with the priest standing-in for Jesus. However, I would think that the Church may make an exception in an emergency situation. Let's say, for example, there was a man in a submarine that was trapped at the bottom of the ocean, and there was no way to rescue this man, but he could communicate by radio with people on the surface of the ocean. In this emergency situation, the Church would likely permit a special dispensation and allow this man to Confess to a priest over the radio, since it would be totally impossible to do in person. But, if it is possible for someone to go to Confession in person, then the Church expects Confession to be in person.

<<<<Okay what about if a person has committed a grave sin and he wants to confess it immediately, instead of waiting for Saturdays where it is the official time for most Parishes in Ghana to hear confessions. >>>>

No, that would still not be an "emergency" situation. If one feels the need to go to Confession before Sunday, then one should go to the church and make a special appointment with the priest, and go to Confession in person. Also, if someone is struggling with serious sin, in this case, it is good to have what's called a spiritual advisor --that is, a priest who knows you and who can work with you to overcome a certain sin or sinful inclination. 
 
<<<<<So I guess the confession I did over phones are invalid? >>>>
 
Well, .... If your priest approved of it, then it is not necessarily invalid, but it is illicit (that is, illegal). There is a distinction. For example, if you meet a Muslim who asks you to Baptize him into the Catholic Faith, you could, technically speaking, take this Muslim down to the river and baptize him yourself. And, if you did that, the Baptism would be valid (it would be a true Baptism), but it would not be licit --that is, you would have violated the Church's canon law; because, according to our canon law, the normal way to bring someone into the Church is to bring the convert to a priest or deacon, and have them Baptize the convert in a public ceremony. Laymen are only supposed to Baptize in emergency situations or when there is no Church minister (priest or deacon) to do the Baptizing. In the same way, if your parish priest gave you Confession over the phone, it may be valid, but it is definitely not licit --that is, the priest was not supposed to do this; especially if he could meet you in person. But, just as with the Muslim in my example above --just as his Baptism would be valid if it was administered by you (a layman) ...because it was not his fault that it was done is an extraordinary way (rather than the normal way called for by canon law), so, if this priest gave you Confession over the phone, his illicit administration of the Sacrament was not your fault, and so your Confession would be considered valid. But, again, this is not the normal way to receive this Sacrament. If you can go to Confession in person, you should go in person (not over the phone). And while I would not worry about being in mortal sin (since you meant well and you did confess your sin to a priest), you should also re-confess this sin next time you go to Confession in person, just to be faithful to canon law. So, do not worry about anything. God is not displeased with you. But, we should observe the Sacraments as the Church directs us to.