Similar situation for us - wanted to give elderly mother a phone, just in case. She "didn't" use the phone much. Note the past tense.
We had a used Android phone for her to use. We added another line ($10 / month on T-Mobile) to our family plan (Unlimited talk and text). T-Mobile set up the phone and didn't even charge us for the sim card for being loyal customers.
My mom then learned and became proficient in texting and calling. But hardly a dent on our family plan. We also installed the free Viber App on her phone. Since she has cable wifi at her home, she can text/chat on Viber for free. She just learned how to Video call on Viber.
By the way, for local and international calling and texting/chatting on Viber. Of course, we've warned her to only use Viber on wifi. Never cellular. I set her phone up that way, just in case. (Viber = cellular off).