We had a similar situation with my step-daughter at the beginning of the summer... we tried all our best tricks (which I will share in a second) but it was still mostly hi/bye... But later in the summer when the novelty wore off and we'd settled out of vacation mode and into a routine, she was more likely to talk.
Okay, here are some phone tricks that have worked at times in the past:
Set up a time ahead of time ("after dinner it will be time to call mom!")
Just before the phone call, remind her of/help her remember things she did that day
Throughout the day if something exciting happens, say something like "wow, I know mommy would love to hear about that! Let's call her later to tell her!" Then later on say "It's time to call mom. Remember you wanted to tell her about..."
Let her know if she doesn't have anything to say, she can ask mom about her day. She could also ask mom to read her a favorite story or sing her a special song.
Try letting her spend time with mom on the phone while she is doing something else, like coloring or playing with her toys or something.
Designate a "special spot" to call from that day... a special cozy chair, [violet] and dad's bed, inside a sheet-and-dining-room-table tent, etc.
Let/Help her pick out a little souvenier or present for mom and call to tell her about the present she got for her.
Stay close by. We don't make a practice of monitoring her calls or listening in, but if it has been a few days since she's had a meaningful conversation, we try to stay close by so we can remind her of some things she wanted to tell mom about or that she can ask mom to tell her about her day.
And, if all else fails, try having her dictate a postcard or draw a picture to send in the mail. Take over the "what I did over my summer vacation" task and send an email or letter yourself (or have your husband do it, though I know that is less applicable in your situation!) My husband likes to send one with lots of photos so she can get a visual, too.
I think sometimes kids don't like talking on the phone because having to recite everything they did that day feels like a chore. In my step-daughter's words, she doesn't like telling my husband what she did at school that day because "I just spent all day talking about it at school and I don't feel like talking about it again." When we gave her the okay to get creative and do things besides recite her day's events, we've had much longer and more meaningful phone calls, and they better serve the purpose of keeping a connection between us.