To dial Paris from outside France, first dial the country code +33 then the code for Paris 1 and then the number e.g. +33 1 123456. To dial Paris from within France, dial 01 and then the number.
Post offices generally operate from 8am to 7pm on weekdays and from 8am until 12pm on Saturdays.
As befits a modern, global city, you can get online just about anywhere in Paris. If you are not using roaming on your tablet or smartphone (do check costs with your network operator before you travel!), then you will be able to access wifi in most hotels, bars, restaurants and even many shops and tourist sites. Remember that wifi will not incur any roaming charges.