When sending alerts through SMS, if the message is longer than 160 chars, it is rejected by the phone.
We used to have an USB "internet key", which did the split itself before sending the message (but required a patch to ignore empty lines before the "CGS" answer from the phone.
We now use a Nokia Asha 300, which rejects too-long messages.
I think using PDU protocol should solve all problems, and is the recommended way.