A lot of what makes a relationship work day-to-day -- reading a face, a tone, catching something in the moment -- gets harder over a phone line or a letter. That makes honest, explicit communication matter more here than it might in a relationship where you're in the same room.
Say the thing instead of assuming it
Tone doesn't always survive a call with a bad connection or a letter written days ago. If something felt off, ask directly rather than deciding you already know what it meant. Most misunderstandings in this kind of relationship come from filling silence with assumptions, not from anything either person actually did.
Boundaries are not the same as distrust
Being clear about what you need -- how often you can realistically talk, what you can and can't take on financially or emotionally, what you need to hear during a hard week -- protects the relationship rather than threatening it. A relationship with no boundaries tends to break under the weight of unspoken expectations eventually; one with clear ones tends to last.
Real concerns deserve a real conversation, not silent resentment
If something is genuinely bothering you -- about contact frequency, about something they said, about anything -- it's worth raising directly and specifically, ideally with time to actually talk it through rather than a rushed minute on a monitored call. Letting it build quietly tends to come out sideways later.
Neither of you can fully know what the other's day-to-day is like
You don't fully know what their day inside looks like; they don't fully know what yours outside looks like. Extending some grace for that gap -- while still being honest when something's wrong -- tends to go further than either partner trying to prove a point.