I'll dig a bit deeper into the apps Papa Sweet mentioned.  

 
SnapChat sucks. Once you read a message it generally disappears. This may be good for opsec, but if you are bouncing between 2 or more POT's/SB's it easy to lose the context of the message thread. If you screen shot a message, she gets a notice and you look suspicious.  

 
WhatsApp and Telegram are better, but... beware that these apps are very popular outside the US as well as within.  Why? because both allow the user to hide their phone number and both use internet to connect, meaning users can talk across the globe for free and without a traceable phone record.  This makes these good for both poor women who can't afford cell service, and scammers - beware.

 
Most US-based POT's will have a US cell phone.  I use a Google voice phone number and the Google Voice app to text.  Pro tip: I also set up my WhatsApp account with my Google Voice number.  

 
Finally, WhatsApp, owned by Meta (ie: Facebook) uses "end to end" encryption. Meaning the message on the phone is encrypted on your device before it gets sent to her, and it gets decrypted only on her device. There is no man-in-the-middle needed to manage encryption/decryption in some central server.  

 
Finally, you will notice that with small devices and small message lengths grammar, spelling and specificity are dying skills.  You need to get used to abbreviations like "wyd" "idk" "ttyl" "dtf" and a ton of emoticons, especially fruit-based emoticons that generally denote sex or certain areas of anatomy.  

 
Feeling old yet?  LOL Just bang a few 20-somethings and you will feel younger.  

 
Life is good

 
The Cat