I can add multiple numbers separated by a text, but it doesn't work to copy and paste multiple numbers. When I try to copy and paste a list of numbers separated by commas, I get the attached error message. Instead, I had to spend an hour copying and pasting them one by one.

I agree with the need for this.

I like RingCentral, but may move
to Textbetter if this is not implemented soon. This can be very simply
done along with allowing us to slightly customize our text message
(such as including the recipient's name). Here are two simple ways we
could do this.

One, allow us to cut and paste a bunch of phone
numbers at once to the RingCentral app for texting. Right now we have
to cut and paste one number at a time, and I just did over 300 of them
on one day. That's asking for carpal tunnel syndrome and there is no
good reason for this.

Two, allow us to upload a spreadsheet or
.csv file with two fields. The first field would be the phone number
and the second field would be the text message. That would easily allow
us to use our own software to customize each message, and then upload
it to RingCentral for texting. This would be my number one request and I
would then be very motivated to encourage other people to switch to
RingCentral as well.

Nice suggestion guys, will definitely try your suggestions and pick one who is the best.