Is the SIM900 3G? Or is the SIM900A able to connect to 3G?
First of all if you are in USA you cannot use SIM 900A since it supports only Dual frequency bands.(900,1800 MHz)it supports only Asian region countries. 

According to Wikipedia, PTel Mobile supports only GSM 850/1900 frequencies. 
so SIM 900 supports( 850,900,1800,1900 MHz ) you must use SIM900

Considering about the speed you cannot obtain 3g, 4g speed since GSM Shield(SIM900) provides maximum of GPRS class 10: max. 85.6 kbps (downlink) so, 

It will able to connect but cannot get such speed in 3g and 4g.