I can understand your concern with getting your phone calls. With RW you can still get them when you don’t have cell coverage at all. That’s only a part of what RW is about. No system is perfect though so a glitch or a software misconfiguration on your phone can send a call to voice mail when you should have gotten it.
This can happen with any carrier but my impression is that it may happen more often with RW than it does with other operators. The other side of that is that this is the place where customers come for help from other customers so that may give me an inaccurate impression of how common it is. Other carriers could be worse and I wouldn’t know or have anything to guide my assessment by.
Texts are unlike phone calls in more ways than may be apparent. Firstly they are not a guaranteed delivery service. In the telephone industry (and similar to UDP messaging in TCP/IP) it means that it can go into the bit bucket instead of to your phone. Again RW has an alternate method for this that involves not only using Wi-Fi if you are out of cell coverage, also text messages go over the Internet rather than the normal/old way where they are forwarded to a server that handles it by forwarding it to your phone or else storing it until you are back on the cell network and sending it to you at that time. “Not perfect” is built in to texting, so there is no carrier that can promise you’ll never miss one.
As @drm186 pointed out cell coverage for the current line of phones and plans will vary depending on which network(s) are strongest in the places you go. RW does have unlimited voice and text roaming at no extra cost which means you can call or text over any roaming partner’s tower that works with your phone (re: GSM/CDMA.) If GSM coverage is poor in your area then you won’t want a Samsung S6 or J3, a Moto Z or and Ascend from Huawei. None of those have CDMA radios.
Hope this helps you in the decision making process. There is also a 14 day money back policy if you want to test the service before committing to it (for a month anyway, there is no contract. )