"This call is not covered by your tariff plan"

[mouseontherun]

Something odd is happening to me. When I'm trying to call someone on his cell phone, having the same area code as me, I'm getting this message: "This call is not covered by your tariff plan" . This is happening with at least three cell phone numbers on different gsm providers, but is working with another cell phone number. I should say that I'm trying to call from a Nokia E63 which is working flawlessly. Before asking, yes, I've tried to call my boss cell phone in the same office with me, same area code with my number. It didn't work. From the same place I've called a friend's cell phone with same area code as me and on the same gsm service provider as my boss, and it worked.
Does any one have any ideas?
Thank you.

[FONGO_kris]

Do the cell phone numbers that do not work happen to be under the same carrier?

If you're not sure you can look them up here: http://www.localcallingguide.com/lca_prefix.php

[mouseontherun]

I've tried about three Rogers and one Telus numbers, all with the same result.
Funny, I can call one of my colleagues which has a company phone in the same plan as my boss. Works with my colleague phone, doesn't work with my boss's phone.