When manually dialing a number in the desktop app make sure to enter the number using the E.164 standard format, including the + sign preceding the number.

Attempting to call an international number without the + sign you will be asked to enter a valid # to call

If you're calling from Salesforce via click to dial, you may also need to add the + sign to the phone number field prior to making the call.

In certain cases if a local phone number format is provided you may need to omit a leading zeroes and reformat the number into E.164 format.

For example the number above may be presented locally as 011 44 025 285 3000, which is not a valid # to call via Truly since it contains leading zeroes and additional dialing codes.)