@ASAP
Troubleshoot by inserting another simcard into iPhone and check whether you have the call failed again.

I'm using Celcom simcard and there is Internet Tethering Tab for me!
Haven't use it though as Celcom 3G charges expensive

I am using OS 3.1.2. I check my Tether before JB and the option is there.

I've change my sim card yesterday but the problem persist! I've done several troubleshooting and I found that, whenever I'm in EDGE area, the call will drop but not 3G. I'm as supp of 1+5 and my principal with iPhone also had this problem using 3.0.1 FW

I think it's a bad combination also. Done all the troubleshooting that I can but to no avail. Still trying though
I've enquire about 64K and 256K simcard, apparently they stop producing 64K already and 256K they said most of the phone still not compatible yet.

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Added on October 13, 2009, 10:40 am@danni_boy
There's several problem same like yours. This issue is on 3.0.X FW so they suggest to update to 3.1.2

This post has been edited by rip_code: Oct 13 2009, 10:40 AM

Base on tweaks, check on brightness, notification, push email, 3g, edge to save abit on battery

A bit update about my drop calls who is using Celcom Postpaid.
Currently, the new batch of sim card that is using code HSUH at the back does not support for 3GS. You may request for 256K 3GWM card and will work perfectly!

Seems that your MMS setting is wrong for DiGi or/and whenever you insert DiGi sim card, your MMS setting remains using Maxis setting.

I'm not really sure for 3G as mine is 3GS using Celcom Postpaid! Having the same issue, 10secs call drop.
Troubleshooting with ON/OFF 3G, EDGE and GPRS but to no avail.
Last thing I checked was the Sim Card, I took friend of mine SIM which is 256K and the problem didn't occurred! (Mine was 128K)
Went to Celcom centre and told them I want 256K simcard but apparently they ain't release the sim just like that. I told them that I troubleshoot something which can solve my problem and thank god they willing to help me out.

Straight away change to 256K simcard and terus test out. No more 10 secs call drop

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Added on January 9, 2010, 1:09 amFYI, I troubleshoot both jailbreak & non-jailbreak iphone.
Meaning this is simcard compatibility problem with my current iphone.

This post has been edited by rip_code: Jan 9 2010, 01:09 AM

No problemo. Just tell them that you've swap with friends sim which is 256K
They told me that 256K will be distribute to user with special case. Then they explain to me as well that certain phone with high tech only will get 256K.
LOL