I have solved, it is not a tinycore problem but a network problem. I have had to write a wvdial config file to connect either gprs or 3g network.
/opt/etc/wind-huawei_e620.conf
[Dialer Defaults]
Init2 = ATZ
Init3 = ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0
Init4 = AT+CGDCONT=1,"IP","internet.wind"
Modem = /dev/ttyUSB0
Phone = *99#
Idle Seconds = 0
Password = CARD
Username = CARD
Modem Type = Analog Modem
Carrier Check = no
Stupid Mode = 1
Compuserve = 0
Baud = 460800
New PPPD = yes
PPPD Path=/usr/local/sbin/pppd
Auto DNS = 1
Dial Command = ATDT
Ask Password = 0
ISDN = 0
[Dialer 3g]
Init1 = AT\^SYSCFG=14,2,3fffffff,0,1
[Dialer gprs]
Init1 = AT\^SYSCFG=13,1,3fffffff,0,0
sudo wvdial -C /opt/etc/wind-huawei_e620.conf 3g &
to connect to 3g network;
sudo wvdial -C /opt/etc/wind-huawei_e620.conf gprs &
to connect to gprs network.
Please note these settings work with huawei internet keys only.