yes, you can use the Opis 60s cable to connect to another country.
Instead of using the "+" button on a mobile phone, you simply dial the international prefix of your country. For most of the EU (including the UK) that is 00, i.e. to dial to France from the UK you would enter 0033 followed by the destination number without any leading 0.
In some countries, the international prefix is different, e.g. in the USA it is 011. So to dial from the USA to France you would enter 01133 and then the local number without any leading 0.