Noctis:
He’d probably be asleep and answer sleepily. But he’d chuckle and stay up for just a little longer to talk with them. He misses them too, but doesn’t get a chance to call very often. So he’d appreciate them calling first. (And honestly, they’d both fall asleep listening to one another’s voices.)
Prompto:
I can picture that he’d be up on his phone anyways when they call, and answer right away. He’d be excited to hear them and he can hear their joy at hearing him too. It’d be infectious and they’d end up staying up all night together on the phone. They’d end up wishing each other a good day before hanging up. (They don’t call very often thankfully, but sometimes Prompto caves and calls them instead which leads to the same situation.)
Gladiolus:
He’d be concerned, and sit up in bed worried. But he’d relax at their answer for why they called, and e’d find it endearing. He’d whisper to them quietly, about the next time he’d be in town, or maybe of their past together. They’d talk for about an hour before making each other hang up, so they’d both be ready for the next day. (He’d call first thing after he woke up to say good morning.)
Ignis:
He’d answer like he always does. But he’s voice would soften and he’d ask how they were upon realizing who it was. They wouldn’t even have to tell him they missed his voice, he’d just know. Because, secretly, he missed their voice as well. They’d stay up together until a rather late hour, just talking back and forth. They’d hang up on one another after promising to call at a decent hour next time.