Three smartphones on T-Mobile with 3GB per phone costs $120/month. You can put the sitter on 1GB and save $10/month. But, you have to buy the phones. You can get the sitter a Moto G for $220. I don't know if you can bring AT&T phones to T-Mobile, but it might work (they're both GSM carriers, but it might not work because something something spectrum frequencies). T-Mobile will pay your ETFs to switch. If you can't bring your AT&T phones, the Moto X is $300 and the Nexus 5 is $350.
Do either you or H work for large companies that might have an employee discount with some carriers?
If you want a land line call your phone company and ask about a metered line. They might have one even if they don't advertise it. We had one 5years ago. You get maybe 50 calls per month and free incoming calls. After your max calls they cost you 10 cents each our something like that.
A landline just seems less likely to get lost or broken.