This is the part nobody warns you about enough: you're allowed to struggle with this too, even though you're not the one incarcerated.
The loneliness is real, not dramatic
You're maintaining a relationship without most of what usually sustains one day to day -- physical presence, spontaneity, just being in the same room. Missing that isn't you being weak or ungrateful for what contact you do have. It's a genuinely hard thing to carry, and it deserves to be treated as such rather than minimized.
Insecurity and jealousy will probably show up
Distance and silence between calls can make your mind fill in gaps with worst-case scenarios -- about them, about whether the relationship can actually hold. That's an extremely common reaction to uncertainty, not a sign something's wrong with you or the relationship. Naming it out loud to your partner, instead of sitting with it alone, usually helps more than either ignoring it or catastrophizing it.
You're grieving a version of the relationship, while still in it
It's possible to love someone and grieve the day-to-day relationship you don't currently have with them at the same time. That contradiction is normal, not a betrayal of your feelings for them.
Take care of the rest of your life too
A relationship that's carrying all your emotional weight, with nothing else in your life to lean on, gets harder to sustain, not easier. Friends, other family, your own routines and interests -- keeping those alive isn't a betrayal of the relationship, it's what makes you able to keep showing up for it.
(See also the pinned guide "For Partners and Spouses: Coping With a Loved One's Incarceration" under Guides → Emotional Support for more on this.)