I checked the fault announcements, but could not find anything related to GSM and GPRS.
Viat ja häiriöt
In general Telia's GPRS seems to be working. I switched to GSM/GPRS for sending this very message.
Have you moved over from the Western, Central or Southern Finland to the Northern or Eastern Finland?
Solid colour: The Finnish Shared Network operating region
In the Northern and Eastern Finland Telia and Dna do not have networks of their own. Instead, you have to use Yhteisverkko, The Finnish Shared Network.
If you have automatic network selection, switching between Telia network and Yhteisverkko should happen automatically as long as you allow roaming on the terminal.
If you use manual network selection, you have to register to Yhteisverkko 244 36 instead of Telia 244 91 when roaming in Northern and Eastern Finland.
This is the case particularly with GSM/GPRS equipment. Yhteisverkko does send pseudo Telia and Dna network identifiers out, but GSM/GPRS equipment generally ignore any pseudo identifiers.
A lot of UMTS equipment and the most of LTE equipment do see the pseudo identifiers, and in this case the terminal works almost as if it was using the home network.
Also, have you checked that the sim is still valid? What happens if you call the number from another phone? Numeroon ei saada yhteyttä, the number cannot be reached is fine, but Numero ei ole käytössä, the number is not in use would indicate that the sim card has expired.
Telia Prepaid sims are valid only 6 months from the date of first use or 12 months from the latest top up, whichever date is later. Telia Prepaid does not have any grace period. When the sim card expires, the balance and phone number associated with the service are lost immediately.