When a Contact's Primary_Phone__c is updated on the primary Contact, sync that phone number up to the parent Account's Phone field.

trigger ContactTrigger on Contact (after insert, after update) {
    Map<Id, Account> toUpdate = new Map<Id, Account>();
    for (Contact c : Trigger.new) {
        if (c.Is_Primary__c && c.AccountId != null &&
            (Trigger.isInsert || c.Primary_Phone__c != Trigger.oldMap.get(c.Id).Primary_Phone__c)) {
            toUpdate.put(c.AccountId, new Account(Id = c.AccountId, Phone = c.Primary_Phone__c));
        }
    }
    if (!toUpdate.isEmpty()) update toUpdate.values();
}