Two options worked for us having this issue on a couple phones with native Android Messages app...
1) Settings > Apps & Notifications > See All apps > Messages > Storage > Clear Storage (won't erase message history, but might help to clean up stale app-related data that could be interfering with messaging).
2) Try using the VZW Messages+ app... it worked for us on the phones having the issues (not ideal, but at least you can text them).
I recently had this problem. I have two phones - a personal phone - Pixel 2 XL on T-Mobile and a work phone - iPhone 7 on Verizon. My Pixel would not be able to receive texts from my work phone on Verizon or several friends who were on Verizon also. I was able to send text messages from my Pixel to iPhone but not the other way around. Finally after multiple calls to T-Mobile tech support, one knowledgeable technician was able to help me. It turns out T-Mobile's back end team had to contact Verizon who changed some settings on their end on how messages were being routed to my personal number and everything got fixed after that. Hopefully this info helps someone out there.
Same issue with my Pixel 3 and one friend. Added the 1 before her number (as suggested in one of the replies) and it worked! Wierd.
Had the same problem (Google Pixel 2 not sending texts to only 1 contact on an iPhone). Fixed by adding the "1" in front of her phone number in Contacts, and also works (without the "1") when using Message+ app instead of standard messaging app that came with the phone. Thanks for all the posts and help!!
It worked!! As of four days ago, my texts were not going through to my sister's iPhone from my Pixel 3 on Verizon, using my default message app. In my contacts I added a "1" in front of her mobile number and presto‼️ My texts are now going through.
Thank you @WILLIESPANK
I was reading some of the other people on this post who were having similar problems. I was running into the same issue with my Wife which is really the only person I NEED to text haha. I added a 1 in front of her number and it worked. Both of us have verizon, both on Pixel 3's.
I've been having this issue for a few days (only when texting my wife). I found that I could send group or MMS messages and she received them fine however, any SMS text would not send. I did as suggested and went to the Contacts entry and selected the general "Text" option. It didn't ask me to confirm anything but did take me to the already open message chain. Once there, I was able to send SMS again with no problem.