Best Cheap Reliable Phone Network/deal And Wifi Query

Hi.

I've checked a few old posts on the topic but i know things change all the time. Basically, i am looking for the best cheap and more importantly reliable network in Bangkok for my partner and i. My partner has used True for a long time but it seems to be getting worse. Friends in Bangkok who call her say the connection regularly cuts off. It's not the phone as it new and she has tried with different phones. She says it's O.K outside but inside the condo it cuts off a lot, particularly when i call from U.K.

So from others experience what would you say is the most reliable network. I heard before the AIS is supposed to be the best but understand this is a lot more money. How does DTAC or others fare? Also something i have found very annoying compared to my network in the U.K is that the service cuts off if you don't keep putting credit on. I use a mobile infrequently. Are there deals that maintain your credit for longer periods?

Also plan to get partner new smartphone and want her to be able to access free wifi in coffee shops etc not bundled network internet access. Will the starter BB model do this or what other cheap models can do this?

Many thanks.