"To use OneSim VoIP, you must have a sufficient Wi-Fi connection, Android 2.0 or higher phone and an active VoIP-enabled Personal Extra Number (PEN) on your SIM card."
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http://www.onesimcard.com/?idmenu=5&...+extra+numbers
If you order an optional Personal Extra Number (PEN), there is a small periodic fee to keep the PEN active based on how you choose to be billed (from $1.67 per month). There is also an additional charge to receive a call through any Extra Number."
for US numbers the charges are:
Monthly $4.99
Quarterly $9.99
Semi-Annually $14.99
Annually $19.99
To all other countries Personal Extra Numbers are more expensive.