Qatar Telecom offers 2 mobile services- Prepaid (Hala) and the regular post-paid one, which is also sold under the brand name Dawli. Both these options usually come with the international mobile roaming facility. If your subscription does not include international roaming, you'll need to apply for it at any Qtel branch.

Check if the 'Network Detection' option on your mobile handset is set to 'Automatic' or 'Manual'.

If network detection is set on 'Automatic' mode, you can simply switch on your handset upon reaching Singapore. Within a couple of minutes you'll get an SMS welcoming you to Singapore from Singtel. And the network indicator on your handset will show you are connected to Singtel.

If you are using the post-paid or Dawli system of Qtel, making calls is very simple- you simply have to dial the number you wish to call with the country code. To call a number in Singapore too, you'll need to enter the Singapore country code.

Such calls are expensive since an international roaming fee is also collected.

If you are using Hala, make sure you have adequate prepaid credit to suffice during the length of your stay in Singapore. Remember, incoming calls are also charged while using the international roaming service. So, dont be surprised if your credit balance starts depleting even though you've not made any calls.

The best thing to do is buy a Singtel mobile card upon arrival at the Changi airport (unless you intend using another form of transport to reach Singapore). These cards are available for SG$25 each and include a small prepaid credit. The prepaid credit can always be topped up at hundreds of streetside shops in Singapore. The most convenient locations are the 7-11 stores in Singapore.

Enjoy your visit.
Cheers!!!