I'm very sorry to hear of your dilemma.
From a legal perspective, there really is nothing that you can do to prevent him from continuing his relationship with his "friend". That being said, if he hasn't given you any other reason to believe that something untoward is going on, then his failure to answer his cell phone when he is with you may be out of respect for the time he and you spend together.
Many people continue friendships that have survived a decade or more after meeting their true love. And, presuming that there is only a friendship at this point, it may be that she has become dependent upon him for plain old conversation. These types of friends really do sometimes speak on a daily basis. It doesn't automatically mean that there is something going on.
Because there is nothing legally
that you can--or for that matter- should do with regard to his relationship with his old friend, you can either see how the relationship works out over time, or if you are fearful that you are being misled, you can chose to issue an ultimatum.
It would be fair that he tell her that he is in a relationship with you at this point. It would seem that if she has been seeing another person, that your new man is not in her sights at this point in time.
Before commingling any funds as a result of this relationship--considering that he has stated a desire to be together with you but not as a married couple, I urge you to enter into a written agreement as regards any financial undertakings you and he might take together--just to be on the safe side and so that each party knows where they stand.
I wish you the best.
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