V1615 no go with Tmobile SIM but OK with O2 SIM

[Guest shirozina]

I want to give my GF my old Vodafone branded TYTN2 (while she waits for her existing contract to run out and buy a G4) When I put in her Tmobile SIM it won't work although it will work with an O2 PAYG sim - is this a Tmobile SIM issue or a handset issue and if the former is there any workaround?