All SIPs the Extension-N VoIP recommends are compatible with WiFi, Mobile 3G, 4G and LTE, and will select the WiFi first if there is a WiFi connection. You can also configure these selections in the SIP on your mobile device.
The Extension-N VoIP system supports codec of G711u, GSM and G729a.
For mobile phone calls only, mostly the Data usage per month for most mobile users is between 50Mb to 200Mb only. If you select G729a codec, it may reduce the data usage between 5Mb to 100Mb. For G729a on any SIP, you need to pay about $10 (one time fee) to the inventor for a lifetime license through the SIP you download on your mobile phone.