The 1100 family of phones don't work via 'standard' AT commands, which is how FrontlineSMS communicates with attached devices. We hope to build a fix for this at some stage since these phones are very popular, but it's not high on the list. Once we release the source code we might be able to get someone in the developer community to work on this for us.
I don't know about the 1600 - this may have the same issue. If anyone else has used one of these I'll let them post any news.