i dont know if this is a true work around or not but i ran into the same problem and what i had to do to get it to finally save the number was i had to save the number as a home number instead of mobile and then after it saved i edited it to a mobile number. i dont know if this is a bug or what but that is what i had to do to add a new number.
There are bugs in this overall lousy contact list. Sometimes, it won't alphabetize by last names, but picks the first instead. Some it won't let you add to faves until you delete it and re-enter. Weird stuff.