Is it a condition for repentance to stop using the phone permanently for someone who is overcome by the temptation to view forbidden things on it, knowing that he uses it for permissible things more than for forbidden ones?

The conditions for repentance from looking at forbidden things are to stop looking at them and to regret what has passed. It is not a condition to abandon the phone that was used in the sin, but abandoning it may help in continuing to repent. If a person fears returning to sin because of the phone, then prudence dictates abandoning it. In fact, abandoning it may become obligatory if there is a strong presumption that it will be used for sin, because the means leading to sin are forbidden. It is possible to replace it with another phone that cannot access the internet.