So I had my k550i + Raspberry Pi set up as an alert gateway for a long time, but incoming never worked. Today I decided to look into it, and with some help from this thread:
http://smstools3.kekekasvi.com/topic.php?id=184
I was able to get it working. So here's my config!

devices = k550i1,k550i2 logfile = /var/log/smsd.log loglevel = 5 #7 for debug admin_to = 436xxx [k550i1] device = /dev/ttyACM0 incoming = yes init = AT+CPMS="ME","ME","ME" #pin = 1111 [k550i2] device = /dev/ttyACM1 incoming = yes init = AT+CPMS="ME","ME","ME" #pin = 1111

Works with k750i.