Last time we were there we picked up 2 generic Motorola phones with pay as you go call credit in Walmart. The purchase cost was $20 each with $30 call credit. We didn't top up once in 2 weeks.
The phones only did calls and texts, no fiddly cameras etc. They suited us down to the ground and as we were staying in a tourist area we handed them to the next family we met who were staying longer than we were.
HTH