I have a Verizon phone and I switched it to Metropcs and I can't send a text message
Which model phone?
Verizon as far as I know has hard coded mms/sms settings in at least one of their Windows phones. The Ativ SE is the one that comes to mind. It was a pain to get it to work on at&t messaging...
If you switched to Metro PCS (T-Mobile mvno) straight from VZ it might still have all the VZW settings only configured...you might get lucky though and can just add the mms apn manually if it's there to edit. If the mms apn is not there you might need to do a hard reset with the sim card inserted but there's no guarantee that could fix it, so use as a last resort...
The sim and network provider settings provision during first boot/setup.
On the icon I have had no issues sending sms/mms on T-mobile. The Ativ SE took some hackery to get it to work on at&t and why I asked which model you have from Verizon.