Their Phone Suddenly Becomes Unreliable
When you call them they are always unavailable because the battery has died or they couldn’t hear the phone because the speaker was not functioning properly. This happens very often and many relationships dissolve because of it, especially at the beginning of the relationship.
So be wary if your partner keeps using their phone as an excuse to not contact you. Try to notice if they are hiding something on purpose by observing if they do that with other people, or is that just something that happens with you.
If they immediately pick up the phone when someone calls them then that might be their paramour. To discover who they are talking to, you might secretly pick up their phone when they are not around and do a little bit of detective work.