Force Messages to use SMS for specific contact rather than iMessage?

iOS 13 and below does not implement a setting on a per contact basis to prefer or force SMS over iMessage/APNS/Messages secure message delivery.

You could do two things to force SMS delivery from iOS.

• disable Messages for the duration of your need to text only via SMS and cellular protocol. All messages will be SMS until you re-enable secure AppleID based messages
• Use a second app that has SMS delivery for the contact in question leaving iCloud messages enabled

To disable iMessages:

1. Open settings app
2. Select messages
3. Tap off iMessage

Then send the SMS to your contact.

I second @bmike's excellent answer:

iOS 13 and below does not implement a setting on a per contact basis to prefer or force SMS over iMessage/APNS/Messages secure message delivery.

However, there's a (slightly clumsy) workaround in that I find helpful in situations like this

Hypothetical: I need to send the message "Go home, don't wait for me; I've changed plans" to a contact in my messages app. This is a perfect use-case for SMS; if their phone is turned on, they'll almost certainly get it within a few minutes. But if we've started to use iMessage then they won't get it until they go home and connect to the internet because I've essentially e-mailed it to them.

I have shared a workaround in an answer here. Here's the workaround:

Force Messages app to send a text message as SMS instead of an iMessage

Here's a workaround that you can use to force the Messages app to send a test message as SMS instead of iMessage to a contact's phone number. It is slight clumsy, but gets the job done in the scenarios like you described:

1. If you have Internet access available on your iPhone via either Mobile data or Wi-Fi (indicated by the presence of appropriate icon in the iPhone's status bar), and the receivers contact number is registered with iMessage, you'll only be able to send an iMessage, irrespective of the online/offline status of the recipient. You can check if a message would be sent as an iMessage by the presence of a blue colored upward facing arrow besides your typed text message.

   

2. To force the message to be sent as SMS instead, temporarily turn off Mobile Data/Wi-Fi on your iPhone to disable Internet access. You'll still only be able to send an iMessage (Blue arrow icon) as your iPhone knows that the receiver is registered with iMessage. Send the message normally.

   The message won't get sent from your iPhone due to the absence of an Internet connection. Now tap an hold on the message bubble, and you'll see an alert at the bottom of the screen with an option that says Send as Text Message. This option is not shown when you have Internet connectivity on your iPhone, or the message is already delivered to the Apple's servers.

   

3. This will cause your iPhone to re-send the message as SMS. The receiver will be able to get the message even in absence of the Internet connection on their device (as long as they have cellular connectivity on their device).

4. Don't forget to turn Mobile Data/Wi-Fi back on on your device to regain access to Internet.

the handiest solution that comes to my mind is to remove your phone number as iMessage adress and use your AppleID instead. To do so go to:

• Settings
• Messages
• Send & Recieve

Deselect your phone number and pick your AppleID for example.

If you want to chat with someone via iMessage, tell the person to use the picked address (in this example AppleID). If you want to text via SMS, they should use your phone number. Onyl one party needs to do this, so your friend doesn't have to.

As of today (iOS 13.2.3) you can't specify/set rules for cases like this within the iMessage system.

Also you could switch to airplane mode before sending out a message, wait for the deleivery to fail (which should immediatly be the case), force to send via SMS and then deactivate airplane mode) + your friend has to do the same sending procidure.