Hey cyn117,
It's really good to know this happens in different locations and with different Wi-Fi networks as it helps to narrow down what's causing this to happen. To continue narrowing this down, we'll need some additional information. Is this happening with iMessages (blue bubbles) or SMS/MMS (green bubbles), or possibly both? Is this happening with all contacts or only one?
In the meantime, be sure your software is updated and then check for and install carrier updates, if available. Updates help by installing bug fixes and security updates. If an update is needed, back up first:
- How to back up your iPhone, iPad, and iPod touch - Apple Support
- Update your iPhone or iPad - Apple Support
- After ensuring you're fully updated, tap Settings > General > About to see if a carrier update appears. If it does, choose to install it. Here's more information, just in case: Manually update your carrier settings on your iPhone or iPad - Apple Support
Keep us posted.
Take care!