This could be due to a number of reasons:

1. Is the service office blocking you from making calls?  Please check if the serviced office is set up on a Fixed (Static) IP or Dynamic IP address? For VoIP to work it needs to be  Dynamic IP address.

2. Does the Serviced office support VoIP i.e. is the router they use VoIP compatible?  What is the make and model of the router? 

3. Can SIP ALG be disabled on the router - this allows the router to be VoIP compatible.

Please contact our Support Desk. with the above information so that we can advise further.