VoIP data usage depends entirely on two factors;
- your choice of codec
- your talk time
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
Also note that although there are lower-tier cable plans available, I wouldn't recommend taking anything with less than 500Kbps uplink, especially if there's a chance you might avail yourself of the "free" (call time only) 2nd line option on many ATA's. Two simultaneous G711u calls will use ~170Kbps of the uplink and many of the Lite
plans are constrained to 256Kbps, which doesn't leave much room for other internet activities while you're on the phone
Finally, before pulling the trigger on the Rogers deal, you might be able to save yourself ~25% by inquiring if Teksavvy cable (Rogers reseller) is available in your area. The added benefit is their Express
plan currently provides 200GB/mo vs. 60GB on Rogers.