We have an XBee that works fine with a SIM we got from Digi. We also have some Verizon M2M SIMs we would like to use, but these do not work very well. The XBee will successfully send an SMS message when it boots, but the pin that is supposed to alternate high-low when the XBee is synced with the cellular network just stays high, and it does not respond to any other messages we send it. Any idea what the difference between the SIMs might be, or how we might further investigate this? Thank you.