When we import numbers from an external integration we apply the default account prefix which is +1.
We use international formatting for numbers (E.164) meaning country code should be specified for contacts with a + symbol, ie +1 for US or +44 for UK.
If needed, we can change the default prefix for your account and re-import the contacts. Please contact Talkdesk Support if you've imported contacts in the incorrect format and need assistance removing and reimporting.