When making an outbound call, the Aircall app will display the recipient's timezone in app:
This timezone is determined based on the recipient's number, and not on the actual geographic location of the recipient.
Different countries have different rules on number series/format distributions. In some countries, landline numbers have fixed series for each state or region, but mobile numbers may not have any regional number series. In other countries, both mobiles and landlines may have regional distribution.
If there is a region-based number series distribution in a country, then the Aircall app will typically display the timezone of that region/number series. In some cases, especially with mobile numbers where there are not region restrictions, the Aircall app may display a different timezone of the country than the one where the recipient is located.