How to make international calls on VoIP?

Generally speaking, dialing international from voip is same as from landline.
i.e. (IDD)+(Country Code)+(area code +phone number)
or
(IDD) +(Country Code) +(03337871389)

Country code for pakistan is 92.
thus: (IDD) +(92) +(03337871389)

IDD, or International Direct Dial Code is dependant on the country from which you are calling from. For dialing international from USA/Canada, the IDD=011. If from the UK, IDD=00.

Thus, from USA/Canada dial:
(011) +(92) +(03337871389)

However, it is possible you may need to drop the zero in front of your number and dial as: 3337871389
in which case, it now becomes:
(011) +(92) +(3337871389)

or
011 92 3337871389 from USA/Canada to Pakistan.