If you're in an area with no data and get cellular only, SMS is all that is going to work. Everything else (RCS/iMessage/etc.) requires data.
VoIP
Rules
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Be civil. Disagreements of varying intensities will happen, but particularly vitriolic attacks will be pruned from the discussion.
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Do not promote or advertise for any business, service or product unless responding to a specific request for recommendations. This includes recommending a user change providers when they have not indicated they are interested in doing so.
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Do not send private messages to users, or invite users to send you a private message, for the purpose of promoting or advertising a business, service or product.
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Do not invite, encourage, or seek help with engaging in unethical or fraudulent activity relating to VoIP, such as call spoofing, robocalling and autodialers, or fraudulent STIR/SHAKEN attestation.
The alternative is reading the rules.