[Serusers] call timeouts

Hi Is there any standard on call timeouts. I hit a really strange problem. I have the following set PSTN ---> SER ---> asterisk All ip devices register to SER Now when a call comes in via PSTN and it destined for a coporate, it hits asterisk, which give its the auto attendant features and all, press 1, and you goto sales, via ser. Now if no one at sales answers, it goes to voicemail in asterisk again. BUT when I make a call from within that company, say to extension 111, which dials out to a IP phone registered in SER, and no one answers, I dont trip to voicemail. After hrs of testing, and a some help from some unknowns on the web, I realised it seemed to be a timeout problem. The call in SER was timing out, and hence call was being destroyed, if I set a timeout in the Dial string in asterisk to a number (which just by luck happened to be less than that in ser) it all worked. Was just wondering if there were any standard timeout parameters which people used for call, cause calls to pstn gateways would take longer than sip to sip ones. Iqbal