I'll share my experience with Speakout. I put my mom on Speakout, as the intention was to minimize the bill, given that she uses limited amount of data or voice calls. She hardly accepts incoming calls calls, and uses Fongo to call out. All has been fine for a few years now until recently. Somehow the account balance just whittled down so fast starting in 2020.
She never had any problem with usage in the past. I can only suspect that calls which go to voicemail, all incoming texts, get charged. I am quite sure this was not the case in the past based on the monthly $ usage in the past. We checked all the call records, but we just cannot add things up properly. Basically, there are too few calls and texts for her to use up almost $40 within 30 days. I just transferred her over to PM, so she will have some data, unlimited incoming calls, as well as unlimited texts.
If the number is used only for emergencies and left off all the time, then Speakout should be really cheap. Otherwise, it's just a pain to keep track of the usage and spot the accounting problems.
