I just tested it and can confirm this behavior. Talk (mobile) app requires you to put the 1. UCP and Sangoma Phone (desktop) don’t.
I tried adding a number rewriting rule in the Talk app and it seems to solve the problem:
- from the keypad, click the gear icon, then Preferences
- Number Rewriting
- (+) to add a rule
- Conditions
- Doesn’t start with: 1
- Length equals: 10
- Actions:
- Prepend: 1
- Conditions
Now when I send SMS to a ten digit number it still looks like ten digits in the messages screen but it goes through, and I can also see the message synced in UCP and Sangoma Phone.