Bell/Telus operate two networks. Each has a legacy CDMA network which is capable of voice and data with CDMA phones purchased from Bell or Telus (or one of their flanker brands).
Additionally they operate a shared 3G HSPA+ network operating on the 850/1900 frequencies. This network supports both voice and data and operates independently of the legacy CMDA networks. They have no fallback to GSM like Rogers so devices on this network must support the right 3G bands. As stated by BGY11, supporting only one of 850 or 1900 may work but with decreased reliability and it's not recommended.
AT&T (US), Rogers, Bell, Telus, MTS, and SaskTel all use 850/1900 for 3G
T-Mobile (US), Wind, Mobilicity, and soon Videotron and Shaw use 1700 for 3G
Additionally they operate a shared 3G HSPA+ network operating on the 850/1900 frequencies. This network supports both voice and data and operates independently of the legacy CMDA networks. They have no fallback to GSM like Rogers so devices on this network must support the right 3G bands. As stated by BGY11, supporting only one of 850 or 1900 may work but with decreased reliability and it's not recommended.
Not true.
AT&T (US), Rogers, Bell, Telus, MTS, and SaskTel all use 850/1900 for 3G
T-Mobile (US), Wind, Mobilicity, and soon Videotron and Shaw use 1700 for 3G