Lots of reasons that could make it not 'odd';
Tinnitus/hearing loss. Not enough to declare at the outset 'Oh, I've got Tinnitus', but enough that calls are very difficult unless both parties are in a totally silent location, the connection is strong, they aren't muffling the microphone with their face, my phone is in exactly the right place that I don't hang up with my cheek whilst trying to hear what's being said, there's a last quarter gibbous moon with the wind coming from a southwesterly direction and the birds aren't coming into roost in the tree outside the window and the other person isn't using handsfree or a shitty phone with dodgy reception. Sometimes hearing issues can creep up on you so you don't realise that you're relying upon a mixture of facial expression and lipreading to support your listening - you just know that phone calls are uncomfortable compared to texts.
Earwigging kids. My old flat was small enough that you could hear everything that went on from all parts. Whilst it could be vaguely amusing that you could hear the bloke downstair's bum squeak as he moved in the bath, it was less amusing when the kids reported to their father that 'Mummy's got a friend' and his usual reaction was less than pleasant. And if they weren't actively listening, the normal levels of noise they made in just existing made it impossible to hear the caller.
Being busy/not wanting to have to talk/finding it intrusive. I'm peopled out at the end of the day and I'm busy during work even ignoring the work rules about mobile usage during the day. Previous boyfriends haven't cared about either and would call repeatedly, demanding my attention then and/or at a set time later that day. I'd answer if it were something important, but once I realised that for some, their 'important' was 'I want attention from you, I'm bored, I know you have your twenty minute break coming up soon when you might speak to a male colleague and not think about me and I'm probably going to demand something else of you as well like sending nudes, the twats ', I readjusted my attitude into 'if it's that important, they'll text me and I can choose whether I am going to answer'. And then you'd be aware that your phone was buzzing with texts and causing you further stress. The lack of respect for boundaries some people showed regarding mobile phones was quite handy in deciding not to continue with seeing them.
Depending upon her age, she might find mobiles a necessary evil rather than this wonderous thing that allows you to be instantly contactable at all times of day, night and in all locations or something that just is. I do miss the times where if I wanted time alone, I could just go out - these days, you don't have that level of privacy/absence of interruption anywhere other than a few remote locations or in a swimming pool where your phone is locked away.
DP always answers the phone to me because he knows if I'm calling him, it's very important and urgent. The rest of the time, text messages for things that are less urgent but can't wait until I get home.
And yeah, she could be married. But if not, for whatever reason, her boundary is that she doesn't like calls but wants texts or talking in person. If you aren't happy with that boundary, you should find somebody else who doesn't have that.