HTC Desire P voice/MMS works, but 3G doesn't
The HTC Desire P does not support AT&amp;T's 3G frequency bands. Its antennas support the 900 and 2100 MHz frequencies, whereas AT&amp;T has deployed 3G on the 850 and 1900 MHz bands.

You cannot use your phone on AT&amp;T's 3G network. If you want to determine if another device would be compatible, you may wish to look at Can I use my device on a different carrier?