At the high end of the scale, the G711u codec (64Kbps) uses about 85Kbps per channel i.e each side of the conversation, which equates to approx 600MB per hour.
I've never checked it, but figure our average 30hrs/month talk time equates to around 18GB usage.
At the lowest end of the scale, the G729a codec (8Kpbs) is the one used for mobile phones and uses 1/8 of the bandwidth numbers given above. However, you might notice a degredation in voice quality.
The bottom line is that even at the highest voice quality codec and your current 13GB/mo usage, your total should be well within the limits of Rogers Express plan.
I've never checked it, but figure our average 30hrs/month talk time equates to around 18GB usage.
At the lowest end of the scale, the G729a codec (8Kpbs) is the one used for mobile phones and uses 1/8 of the bandwidth numbers given above. However, you might notice a degredation in voice quality.
The bottom line is that even at the highest voice quality codec and your current 13GB/mo usage, your total should be well within the limits of Rogers Express plan.