Don't assume anything yet. You have to get home before you assume that it is the relationship. Often in business the first 3 years or so are just about stress. There is often a fear of failure that last for this time. He may be experiencing stress in his business and feels overwhelmed. This could lead him to not want to talk about it on the phone. If you assume it is about the relationship then you will think negatively when you don't know that is what it is.
If it is the relationship you can isolate the cause of the problem. It doesn't mean that it is over because there is an issue. You approach the problem together and most things are fixable. The point of talking about it is to stay together. If he doesn't share his emotions then this would be a good time to discuss that. A great resource on this is Mars and Venus Together Forever. He has to learn how to do this. It takes practice. He has lot of time to look at that quality if he is home alone and can explore this. Find ways to increase his ability to do this. For instance have him send an email each day you are away to compose his thoughts. Then transfer this skill to text then phone. If he is committed to the relationship then he is motivated to maintain it. You may tell him now that you need to just know what the problem is in reference to so that you don't have difficulty functioning. He may say it's about his business. Then you can pick it up when you get home. If he refuses then still don't over analyze this so that you can't function.
If it turns out that it is about the relationship, offer to go to couples counseling to resolve the issue.
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