Which Modem is reliable for sending / receiving SMS with kannel?

Hello,

may i introduce myself, i am Georg from Koblenz / Germany and i am using
kannel for years for sending SMS-alerts.

I am using kannel version 1.4.3 and a Siemens TC35 Modem with a SIM card
from the german Telekom. We receive about 3 SMS every 5 minutes, which
works fine normally, but about every two days or so, the modem stops
receiving SMS messages. It is still logged into the mobile network
which is indicated by the flashing of the green led and it still does
send messages, but no messages are received. If i then power it off and on
again and restart kannel, it receives all the messages that have been
missing before.

I phoned the hotline from Telekom and they could see that the messages
are not delivered sometimes and they suspect a bug or defect in the
modem and i should probably try a firmware update or another modem.

Thus finally my questions: What do you think of this? Which type
of modem is reliable for sending and receiving SMS? Has anyone ever done
a firmware update on a TC35?

Kind Regards,
Georg