--[[
example connect in /var/log/syslog:

Apr  1 20:06:44 stefan-Aspire-3810T kernel: [  281.020329] usb 2-2: new high-speed USB device number 6 using ehci-pci"
[...]
Apr  1 20:06:44 stefan-Aspire-3810T kernel: [  281.153752] usb 2-2: Product: Cynus E1

example disconnect in same log:

Apr  1 20:07:23 stefan-Aspire-3810T kernel: [  319.671170] usb 2-2: USB disconnect, device number 6

phone in output of "usb-devices":

[blank line]
T:  Bus=02 Lev=01 Prnt=01 Port=01 Cnt=01 Dev#=  8 Spd=480 MxCh= 0
D:  Ver= 2.00 Cls=e0(wlcon) Sub=00 Prot=00 MxPS=64 #Cfgs=  1
P:  Vendor=0bb4 ProdID=0003 Rev=02.55
S:  Manufacturer=MediaTek
S:  Product=Cynus E1
S:  SerialNumber=0123456789ABCDEF
C:  #Ifs= 2 Cfg#= 1 Atr=c0 MxPwr=500mA
I:  If#= 0 Alt= 0 #EPs= 1 Cls=e0(wlcon) Sub=01 Prot=03 Driver=rndis_host
I:  If#= 1 Alt= 0 #EPs= 2 Cls=0a(data ) Sub=00 Prot=00 Driver=rndis_host
[blank line]

]]

get("#515") -- splitLines

bla = os_backtick('usb-devices')
lines = splitLines(bla)

for _, line in ipairs(lines) do
  if line:match("S:%s+Product=Cynus E1") then
    print("Phone connected!")
    return
  end
end

print("Phone not connected!")